As law enforcement flocks to the scene of the bombings at the Boston Marathon, the information they are able to gather at the scene of the attack will be critically important in identifying suspects and building a criminal case against them. Carefully gathering evidence will facilitate criminal prosecution and will aid in successful scientific tests and analyses. The response will likely unfold in the following steps, some of which may occur simultaneously.
FIRST RESPONSE STAGE
On scene responders and others who arrived shortly thereafter were likely immediately concerned with helping those who were injured. As the scene was brought under control, they likely shifted their attention to assessing the situation, identifying emergency services required, spotting safety concerns and noting evidentiary concerns. To manage personnel, first responders will need to establish a command post, the primary place from which additional personnel, bomb techs, EMS, and others will be requested and dispatched.
At this stage first responders will also identify ongoing hazards, buildings that should be evacuated, structures that are at risk of collapse, and the possibility of secondary explosive devices--a challenging task given the number of bags and other personal items discarded at the scene. Law enforcement will also face a challenge attempting to identify witnesses and evidence--especially "transient physical evidence" (residue, chemicals, etc. that may be on victims or their clothing and could be compromised as victims and witnesses leave the scene). They will also identify the possibility of chemical or biological hazards.
The paramount concern at this stage is safety, so the search for secondary devices will take precedence over all other concerns. Evacuations, detonation of suspicious packages, cordoning and controlling of hazardous areas and establishment of safety zones will all be standard practice at this stage of the response process. First responders will also establish a perimeter, staging areas and will begin the process of documentation.
PRELIMINARY SCENE INVESTIGATION STAGE
Once the incident was confirmed as a bombing, investigators likely arrived on the scene and reported to the incident commander. While press briefings were ongoing, first responders, investigators and other personnel were similarly briefing essential personnel. Investigators at this stage will be concerned with ensuring the scene is safe, secured, and the risk of secondary explosive devices has been minimized.
Given the number of first responders, volunteers, and other personnel who are unfamiliar with a crime scene investigation, investigators will need to ensure that all personnel are advised of the need to prevent contamination of the crime scene. Moreover, as people identify possible evidence (shrapnel, unexploded devices, etc.) a chain of custody must be established for evidence that was collected and will be collected. To prevent contamination of the crime scene, investigators may establish procedures to document who enters and exits the crime scene and may even establish a single entry and exit point. They will also want to establish procedures for ensuring the scene is not contaminated and those procedures will need to be documented, other documentation should also detail the procedures for evidence collection and chain of custody.
As things begin to settle down, law enforcement should consider using "consent forms" for anyone who is searched for evidence related to the bombings. Investigators will also want to conduct a second walk-through at this stage, identifying safety concerns, possible transient evidence, and will want to identify the specific location ("the blast seat") of the explosions. Investigators will identify additional resources, and various agency officials will begin transmitting requests throughout their agencies for personnel, equipment, and other resources requested by investigators. Lists of victims and witnesses will be developed and their individual accounts of the incident will be documented.
SCENE DOCUMENTATION STAGE
As first responders gain control of the scene they will need to begin documenting everything they and others see, hear, and know about the situation. They will need to document access to the scene (who is coming and going and when), and they will need to document the timeline of the incident and investigation. Investigators will begin to compile a narrative report describing the overall scene, physical conditions, odors, structural conditions, etc. They will also create a diagram of the scene, labeling features using maps and engineering drawings supplemented by GPS, surveying equipment and other technology. This documentation will form a permanent record of the incident, and will aid in crime scene reconstruction.
At the same time that the narrative report is being generated, other investigators will be gathering photographic and video documentation of the scene --- ideally before items are removed or disturbed. Investigators will also request footage and photos from witnesses who may have documented the incident and the reactions of people at the incident. That evidence may provide additional angles that supplement evidence the investigators already possess --- as they study this footage they may also look for people who were behaving suspiciously or who didn't seem surprised by the event. By combining multiple pieces of video evidence, investigators will be able to create a view of the scene and surrounding area that will allow evidence and items to be spatially related.
Investigators will also seek to interview those persons who come forward with evidence as they are not only providing evidence but are also witnesses. Investigators will also seek other witnesses who may have fled the scene or were transported from the scene and will try and prioritize the interviews from the hundreds of potential witnesses. Those interviews will focus on the timeline of events, injuries the witness may have suffered or observed, their relationship to victims, and the basis of their knowledge about the attack. Victim and witness statements are oftentimes essential pieces of evidence in establishing the circumstances of the incident, potential perpetrator, and the nature of the device.
EVIDENCE COLLECTION AND PRESERVATION STAGE
Given the size of the crime scene(s) and the number of witnesses, collecting evidence and preserving it will be a difficult task for investigators. The investigative team will include forensic specialists, medical examiners, photographers, structural engineers, bomb disposal technicians and analysts, sketch artists, and others looking for and collecting evidence. All of these individuals will need to be instructed on procedures for gathering and preserving evidence, and throughout the process a lead investigator will review practices and procedures ensuring investigators are adapting to changes and unforeseen circumstances.
Given the fact that there are two bomb sites, there seem to be two interconnected crime scenes. Thus procedures will need to be established to minimize cross-contamination --- such procedures might mean packaging and transporting evidence from different locations in different containers, using protective outer-garments that are changed from scene to scene, and maintaining "control samples" such as empty evidence collection containers, swabs of personnel and swabs of equipment used. The goal of these procedures is to link evidence of the bombing to potential suspects, protect evidence, minimize contamination, and preserve the chain of custody.
REOPENING THE CRIME SCENE
Once the investigation is completed and documented, the crime scene can be reopened. Before that can happen the lead investigator will review all documentation including the activity log, interviews, narrative description of the scene, diagrams, maps of evidence collected, videos, and photos. If at any stage in this review the lead investigator deems some piece of evidence is missing or an issue has not been documented, the scene may stay closed until the lead investigator is satisfied that the issue has been addressed.
The lead investigator's review of documentation is generally conducted in a team fashion with critical questions being raised by those who were present at the scene, and others who were not present who may be able to identify issues that investigators may have overlooked. The lead investigator will likely release the scene to another agency, rather than reopen it himself/herself. That agency will likely address public health concerns such as biological hazards (blood or other bodily fluids), chemical hazards, and structural safety issues (buildings at risk of collapse). All of the information gathered will be subjected to laboratory analysis, and technical information about the explosive devices will be cross-referenced with national databases.
HOW LONG UNTIL A SUSPECT IS IN CUSTODY?
There's no way to know how long it will take investigators to find the person responsible, although after the Oklahoma City Bombing and the failed Times Square Bombing, suspects were caught within days of the incident.
Gregory S. McNeal is a professor specializing in law and public policy. You can follow him on Twitter @GregoryMcNeal.
