Post-Blast Scene: Search Grid, Fragment Collection, Seat of Blast

The first arrival at a post-blast scene is almost always an emergency responder, a police officer or paramedic, who has no specialised explosives training and whose immediate objective is casualty management, not evidence preservation. The forensic post-blast investigator does not arrive first. By the time the investigator enters the scene, it has already been partially disturbed by emergency access, firefighting (if a fire followed the blast), and triage of casualties. This is the operational reality the methodology must accommodate.

Before any forensic personnel enter, a bomb disposal or explosive ordnance disposal (EOD) team must conduct a render-safe assessment of the scene for secondary devices. The practice of planting a secondary device targeted at first responders and investigators has been documented in IRA operations in Northern Ireland in the 1970s and 1980s, in Taliban IED operations in Afghanistan and Pakistan from 2001 onward, and in isolated domestic terrorism incidents in the United States and Europe. The UK's 321 EOD Squadron, the US Army's 52nd Ordnance Group EOD, and India's Bomb Detection and Disposal Squad (BDDS) units all have formal protocols for secondary device assessment before they clear a scene for investigation.

The cordon structure, once the scene is cleared, typically has two concentric boundaries. The inner cordon encompasses the primary damage zone (the crater, the near-field brisance zone, the area where evidence density is highest). Only personnel with assigned evidence-collection roles and a specific reason to enter should cross this boundary. The outer cordon encompasses the broader area where fragment scatter, far-field overpressure damage, and witness-accessible ground are controlled. Between the cordons, a scene log records every person entering and exiting the inner area, with time stamps and role designations, providing the chain-of-custody foundation for every exhibit collected within.

Photographic and videographic documentation of the scene before any collection begins is mandatory. Aerial or elevated photography, from a cherry picker, adjacent building, or drone (where legally authorised and operationally safe), captures the full spatial extent of the damage and the locations of visible significant items before foot traffic disturbs them. Ground-level photography documents the orientation of displaced objects, the direction of blast-ejected material, and the geometry of any visible crater.

Post-blast search methodology applies the expanding concentric grid as its primary spatial framework. The grid is centred on the probable seat of blast, identified initially from the most severe visible damage (the crater, the zone of heaviest structural destruction, the point of fire origin if a post-blast fire occurred). From this centre, concentric rings are laid at measured radii, typically at 5 m intervals in the inner zone (where fragment density is highest) and 10 m or 20 m intervals in the outer zone.

Each annular sector between concentric rings is divided into segments by radial lines, producing a grid of cells analogous to sectors on a clock face. Each cell receives a designation (Ring 1, Sector A; Ring 2, Sector C, etc.) that becomes part of the exhibit reference for every item found within it. The cell designation is recorded on the bag seal, the field log, and the exhibit database. A fragment found in Ring 3, Sector D without a precise coordinate is of limited forensic value; the same fragment recovered with its grid cell, bearing, and approximate depth above or below grade becomes a data point that contributes to the directional reconstruction.

Cell dimensions in the innermost ring are kept small enough that an examiner on hands and knees can cover the area systematically in one pass, typically 2 m x 2 m or 3 m x 3 m. In practice, examiners use string lines, chalk marks on hard surfaces, or surveying pegs to mark cell boundaries before beginning collection. A designated recorder accompanies each collection team, logging every item found, its grid location, and its orientation before it is bagged.

In large-scale scenes (vehicle bomb, building collapse, aircraft wreckage), the grid may cover hundreds of metres of radius and require multiple teams working in parallel sectors simultaneously. The 2002 Bali bombings at the Sari Club and Paddy's Bar required a 300 m radius search grid that extended across roads, collapsed structures, and public areas, coordinated by the Australian Federal Police with Indonesian Polri forensic teams, with cells assigned to specific agency teams and a central evidence receipt point at the cordon boundary.

The innermost area, the 5 m radius directly around the crater, is treated with special intensity. Here, the explosive residue concentration is highest, the microfragment density from device components is highest, and the soil profile beneath the crater surface holds layered information about how the device was positioned. Soil sieving (dry or wet sieve to 1 mm mesh) at the innermost zone is standard practice under UK SSPBI and ATF post-blast guidelines, recovering component fragments too small to be visible in a visual search.

Fragment collection in a post-blast scene follows a protocol that is slower and more methodical than normal crime-scene evidence collection, because the spatial information attached to each fragment is as important as the physical specimen itself. The protocol applied by the UK Forensic Explosives Laboratory and the ATF specifies that every recoverable fragment, regardless of apparent significance, is individually bagged at its point of discovery, with the following information recorded before the bag is sealed:

The grid cell reference (ring and sector designation, or UTM coordinate if GPS logging equipment is in use). The orientation of the fragment as found, which is typically recorded as the bearing of the longest axis and whether the item was face-up, face-down, or embedded. For fragments embedded in structural material (a wall, a floor, a vehicle panel), the angle of penetration below horizontal is recorded because this constrains the elevation of the seat of blast relative to the structure. The depth of the fragment below grade or above grade, in centimetres, referenced to a datum. The material type, initially as a visual assessment (metal, plastic, fabric, electronic component, organic), to guide triage at the evidence receipt point.

Metal detector sweeps precede and follow hand searches in each grid cell. Standard forensic metal detectors used in post-blast contexts (Garrett Pro-Pointer AT, Fisher F-75, or equivalents as specified in local SOPs) respond to ferrous and non-ferrous metals. For IED components, detonator wire, switch components, and battery terminals are among the highest-value targets. In terrain that itself contains metal (reinforced concrete, corrugated metal roofing, metal-pipe water infrastructure), discrimination mode and frequency selection on the detector require operator training beyond the basic level.

Sieving at the crater and innermost ring is a separate discipline. Collected soil and debris from the crater is bagged by depth layer (0 to 5 cm, 5 to 10 cm, below 10 cm), brought to the evidence examination point, and dry-sieved through a 2 mm mesh, then a 1 mm mesh. Material retained on each sieve is examined under a portable inspection light. Wire fragments as small as 3 to 4 mm (consistent with detonator fuse-head wire), granular explosive particles, circuit board fragments, and micro-plastic shards have been recovered from post-blast sieve residues at the Oklahoma City bombing (1995), the 7 July London bombings (2005), and multiple IED scenes in Afghanistan and Iraq under CEXC (Combined Explosives Exploitation Cell) protocols.

Post-blast explosive residue is present at the scene in concentrations that fall off steeply with distance from the seat and with exposure to heat, rain, and foot traffic. Collection strategy must prioritise locations where residue has been driven into or trapped in a protected microenvironment and is likely to have survived. The standard hierarchy of collection points, based on research by the Forensic Explosives Laboratory (FEL, Porton Down), the Swedish National Forensic Centre (NFC), and the FBI Laboratory Explosives Unit, is as follows:

Sealed crack surfaces within the crater and the innermost 2 m around it. Detonation overpressure drives gas products and unreacted explosive particles into existing cracks in concrete, masonry, and asphalt at the instant of detonation, before the heat of the fireball can decompose them. Swabbing the interior face of a freshly opened crack (not the outer, exposed face) recovers the highest-concentration residue. In practice, an investigator uses a chisel or cold knife to open a crack that was sealed, then swabs the interior surface before air exposure has occurred.

Underside surfaces of structural elements directly above the seat. Floor slabs, ceiling panels, beam undersides, and vehicle underbodies that were shielded from rain and foot traffic but directly in the path of the detonation gas products accumulate and retain residue. Swabbing with a dry cotton swab first, then a wet swab (methanol or acetonitrile, solvent grade), is the standard collection method per the ATF and FEL SOPs.

Protected voids: areas beneath floor tiles that were dislodged but not inverted; the interior of hollow structural members (box sections, H-beams); drainage channel interiors that captured flowing residue-bearing water before it dispersed.

In India, the CFSL (Central Forensic Science Laboratory) operates under guidelines aligned with Interpol IEPA (Illicit Drug and Explosive Programme) standards for swab collection; state FSLs vary in protocol quality. The UK's SSPBI framework formally defines collection priority zones and requires documentation of the collection location (grid reference, surface description, protected status) for every swab exhibit. US ATF guidance, implemented through joint ATF/FBI post-blast training delivered at the Redstone Arsenal Forensic Sciences Laboratory, specifies swab type, solvent, and container requirements.

Swab exhibits are containerised in glass vials (not plastic, which may absorb organic traces) with PTFE-lined caps, labelled with exhibit number, collection location, date, time, and collector identity, and submitted to the laboratory under chain-of-custody documentation. At the laboratory, GC-MS analysis for organic explosives (TATP, RDX, PETN, TNT, NG, EGDN, HMTD), ion chromatography for inorganic residues (ammonium nitrate, potassium nitrate, perchlorate), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) for elemental and particulate characterisation are the core analytical tools.

The crater is the most visible indicator of the seat of blast, but not the only one, and in some scenarios (a device suspended above a floor, a vehicle bomb that burned post-blast, a blast in soft soil that collapsed the crater walls) it may be distorted or obscured. Investigators use at least three independent lines of evidence to locate and confirm the seat.

Crater geometry and material analysis provide the primary fix. In a bare earth or asphalt surface, a detonating charge produces a roughly circular crater whose diameter and depth depend on charge mass, depth of burial, and material properties. For a known explosive type and burial depth, back-calculation of charge mass from crater dimensions uses empirical relationships validated by US ERDC (Engineer Research and Development Center) and UK DSTL (Defence Science and Technology Laboratory) test series. In hard structural materials (reinforced concrete floor slabs), the crater appears as an inverted cone of spalled material on the underside of the slab (Hertz contact spallation), and the geometry of the cone constrains the device position relative to the slab. Soil removed from the crater during sieving is also characterised for exotic chemical or physical signatures (remnant prills from AN, plastic binder residue) that confirm the explosive type.

Crater glass, glass fragments found within or immediately adjacent to the crater, often originates from the device container itself (a glass bottle or pressure-vessel used to house the explosive) or from glazing immediately adjacent to the seat. Crater glass is distinguished from far-field blast-damaged glazing by its size (much smaller, often powder), its thermal treatment (some glass in the immediate blast zone shows thermal remelting on one surface from the fireball), and its chemical composition (soda-lime float glass vs borosilicate vs toughened laminated). Paint layering on glass shards, recovered in the innermost zone and compared against reference paint samples from surfaces near the seat, can place the seat within the footprint of a specific room or vehicle.

Paint transfer analysis is particularly powerful in vehicle bomb scenes. A vehicle bomb that detonated while in contact with a road surface, adjacent to a wall, or inside a parking garage leaves paint transfer on nearby fixed surfaces from the displacement of the vehicle body. Paint layers (primer, base coat, lacquer) from the bomb vehicle are deposited on concrete, masonry, or other vehicles in the near field. Forensic paint comparison by Fourier-transform infrared spectroscopy (FTIR) and pyrolysis GC-MS can match the layer sequence to a vehicle make, model, and production year, narrowing the vehicle identification before any VIN or registration investigation. This technique was applied in the 1993 World Trade Center bombing (US) and the 1996 IRA Manchester bombing (UK) to establish vehicle make before any registration evidence was available.

Witness marks in surrounding structures are the triangulation element. A fragment embedded in a wall, a column, a vehicle body, or a tree at a known position and orientation defines a vector back toward the seat. Two such vectors from different positions define a point; three overdetermine the point and quantify the uncertainty. The depth and angle of penetration constrain the fragment velocity and hence, via the fragment distribution model, the standoff from the device to the struck surface. Combined, multiple witness marks in different structures produce a three-dimensional reconstruction of the seat that is independent of the crater (and therefore valuable in cases where the crater has been disturbed or is absent).

Major post-blast scenes are invariably multi-agency in a way that routine crime scenes are not. In most jurisdictions, three distinct command structures converge simultaneously: the emergency services command (fire, ambulance, structural engineers) whose primary concern is casualty recovery and building safety; the law enforcement command (police outer cordon, intelligence coordination, suspect management) whose concern is the criminal investigation; and the specialist forensic/EOD command (post-blast investigators, forensic scientists, EOD teams) whose concern is evidence. These three command structures have competing access requirements that can be managed only through a pre-agreed coordination protocol, not improvised on the day.

The UK uses the JESIP (Joint Emergency Services Interoperability Programme) framework, which mandates a joint decision log maintained by co-located commanders from each agency from the moment of initial response. The designated lead investigator (typically a senior Detective Superintendent or Counter Terrorism officer) has primacy over evidence recovery decisions once the scene is rendered safe. In the United States, the FBI has operational primacy in domestic terrorism post-blast investigations under 28 U.S.C. Section 533, with ATF providing the lead forensic capacity. State and local law enforcement operate under memoranda of understanding that define access protocols and exhibit custodianship. In India, the National Investigation Agency Act 2008 gives the NIA primacy in terrorist blast investigations, with BDDS and CFSL providing EOD and forensic capacity; state CID or CBI may operate in parallel depending on the jurisdictional classification of the incident.

The 2008 Mumbai attacks (26 November) involved IEDs, improvised grenades, and firearms at eleven sites simultaneously, requiring coordination between the Mumbai Police, the Maharashtra ATS, the NSG (National Security Guard), BDDS, CFSL, and later NIA, across active crime scenes, siege sites, and post-blast scenes over more than 60 hours. The coordination failures at some sites, where scenes were accessed before render-safe clearance, are documented in the subsequent NIA charge-sheet and government inquiry reports and have driven reforms in India's multi-agency blast investigation SOP.

The 7 July 2005 London bombings produced four simultaneous underground and bus blast scenes at Edgware Road, Aldgate, King's Cross/Russell Square, and Tavistock Square. The Metropolitan Police Counter Terrorism Command (SO15), the FEL Porton Down, and the British Transport Police coordinated across four sealed crime scenes while the London Underground network was shutting down and 52 fatalities and 700 casualties were being managed. The scene management lessons from 7/7, formally reviewed in the Intelligence and Security Committee report of 2006, directly shaped the current SSPBI framework and the scene log and exhibit handling protocols that the UK now trains allied forces in through the College of Policing Post-Blast Investigation programmes.

1. Command co-location and scene log initiation
   Establish a joint command post at the outer cordon. Initiate scene log recording every entry/exit from the inner cordon. Assign lead investigator with primacy over evidence decisions once EOD clearance is received.
2. EOD render-safe and primary survey
   EOD team conducts systematic sweep for secondary devices. Primary survey photographs the scene before forensic entry. Structural engineers certify building safety if applicable.
3. Aerial/elevated overview documentation
   Drone survey or elevated photography documents the full damage zone, visible fragment distribution, and crater geometry before ground-level collection begins.
4. Grid establishment and sector assignment
   Lay the concentric search grid from the probable seat. Assign sectors to specific evidence teams. Brief each team on collection protocol, bag labelling, and exhibit receipt procedures.
5. Inner-zone intensive search
   Work the innermost two rings on hands and knees with metal detector and sieve. Collect all fragments. Swab crack surfaces and protected undersides for residue. Sieve crater soil by depth layer.
6. Outer-zone systematic search
   Work outer rings by walking search and metal detector sweep. Collect all embedded fragments in structures. Document witness marks with bearing, angle, and depth measurements.
7. Exhibit receipt and triage
   Centralise all exhibits at the cordon evidence receipt point. Assign sequential exhibit numbers. Complete continuity documentation before any exhibit leaves the scene. Screen key swabs with field IMS if available.
8. Scene handback
   Complete a final photographic survey after collection. Formally document the handback of the scene to emergency services or building owner with a signed scene log. Archive the grid maps and exhibit logs as permanent case records.