What is involved in the examination of a fire scene?

We determine how a fire started

Put simply, the role of the fire investigator is to determine how a fire started. However, the aim is to answer a few key questions that will hopefully allow the exact cause of a fire to be identified. Firstly, the investigator will try to pinpoint where exactly a fire originated, secondly what the source of ignition was, thirdly what the item was that first ignited and lastly, how it was that the ignition source and the item that ignited first came together.
Depending on the scale of the fire and the amount of resultant damage, the job of cause determination can either be relatively simple or extremely challenging.

A fire investigation will often begin well before a fire investigator even arrives at the scene of a fire. Information can be gathered from weather reports in the event of a lightning strike for example, or intelligence on a particular address might prove useful if previous fires have occurred there.

Fire witnesses can be critical

Once on scene, fire investigators will identify as many key witnesses as possible to obtain their actions and observations prior to and during a fire. Potential witnesses include building occupants, neighbours, passers-by and fire crews who may have seen something of vital importance. Whether firefighters arrived and had to force an entry to a secure building or conversely found signs of a prior break-in for example is of vital importance to the investigator. Witnesses however may not be truthful, may exaggerate or make assumptions that must be taken into account. The physical examination of a fire scene involves surveying not just the area where the fire has damaged the building, but also rooms or areas remote from that location.

Fire Investigation Service

Where a fire originated

We also gather lots of information relating to the general housekeeping and habits of the occupants of buildings, such as carelessly discarded cigarettes, overloaded electrical extension leads and the presence and use of candles. Having gathered as much information as possible, the investigator will then assess and interpret the fire patterns that may be present. Soot patterns allow investigators to determine the extent of smoke travel inside a building and work out where the fire was at its most intense. Soot adhered to walls can be burned off by direct flame impingement and so ‘clean burn’ patterns can prove extremely useful.

The degree of charring to wood, the moisture loss from gypsum plaster board and the presence of electrical arcing on circuits are just a few of the many techniques that allow a fire investigator to ‘home in’ on where a fire originated. Fire debris must be carefully handled as copper electrical conductors for example will be brittle and are easily damaged. It is for this reason that a fire scene examination is a very slow but thorough and methodical process, and commonly involves the use of small brushes, sieves and magnets; it can be the tiniest piece of debris that allows the investigator to identify the cause. In a similar way to archaeology, fire debris piles into layers as it falls to the floor and forms a timeline and it is the lower layers that often contain that key piece of evidence. Sources of ignition that are often found even in the most severe of fires include electrical components of failed appliances, tea-light candles carcasses and cigarette lighters.

Arsonists can incorrectly believe that ignitable liquids used to accelerate deliberate fires will be entirely consumed; this is very rare and residue from such liquids more often than not will still be present after a fire. Fire Investigators must assume nothing, believe nobody and check everything! People can lie whereas evidence does not and it is absolutely crucial that investigators maintain an open mind. They must consider all possibilities and check that fire cause hypotheses stand up to scrutiny and are robustly ‘tested’. Ultimately, a fire scene investigator has one overarching responsibility and that is to establish the truth and remain completely impartial throughout any investigation.