The forensic utility of soil - scientific analysis of soil aids in criminal investigations

From 1990 to 1992, investigators with the New York City Police Department (NYPD) conducted a joint investigation with the FBI, the Drug Enforcement Administration, and the Bureau of Alcohol, Tobacco and Firearms into an organized crime family. An element of the investigation involved the shallow burial of five murder victims on Staten Island. NYPD investigators forwarded digging tools seized as evidence during the investigation, as well as soil samples, to the FBI Laboratory for examination to determine whether the implements were used to bury the victims.

Investigators packaged two shovels and a pick separately, ensuring that brown paper bags sealed with evidence tape protected the blade and head portion of each tool. They also selected known soil samples from each of the five graves, based on noticeable color changes in the soil profile. (Differences in soil composition and texture generally manifest themselves through changes in soil color.) Investigators packaged these soil samples in labeled 35 millimeter (mm) film canisters, Additionally, they drafted a dimensional crime scene sketch that depicted rave locations and relevant landmarks.

The map assisted personnel from the FBI Laboratory in providing additional investigative assistance. While on site, Laboratory personnel collected additional soil samples taken randomly at distances ranging from 100 yards to approximately one-half mile from the gravesites. The personnel also collected "alibi" samples--specimens that could confirm alternate and legitimate sources of the soil. These came from two residences where the shovels and pick could have been used for gardening or other purposes.

Prior examination of the tools revealed a small amount of soil (one-half of a film canister) from one of th hovels suitable for comparison. Soil samples recovered from the other shovel and the pick were contaminated by oil and rust, thereby limiting their forensic value.

Based on color, texture, and composition, Laboratory examiners determined that the soil recovered the shovel shared characteristics with the soil the burial sites. Conversely, gross dissimilarities existed between the soil on the shovel and that collected at the residences, effectively eliminating those are s as possible sources of the soil. During two separate trials, expert testimony regarding the soil samples contributed to the conviction of two principal members of the organized crime family.

FORENSIC VALUE OF SOIL

This case demonstrates the potential forensic value of soil when investigators properly collect, preserve, and package evidence before forwarding it for laboratory examination. Sometimes, attempts to exploit the forensic benefit of soil analysis meet with limited success, due to improper evidence collection and documentation. To ensure the best possible results, investigators are reminded to appreciate the nature of soil and follow certain guidelines when collecting documenting and forwading soil samples, tools, and related items to the FBI Laboratory for examination.

The Nature of Soil

Soil can generally be considered the natural accumulation of weathering rocks, minerals, and decomposing plants. The formation of soil represents a dynamic process, influenced by a number of factors, including climate, geologic parent material, relief, biological activity, and time. Soil may develop in place (in situ) or after being deposited by wind, water, animals, or human activity.

Additionally, and of particular forensic significance, soil may contain materials produced by humans, such as brick fragments, roof shingle stones, paint chips, glass, and other items. Because these materials improve characterization, they may strengthen the association between specimens.

Soil varies laterally--that is, across the land surface--from place to place. These changes may be abrupt, occurring within a few meters, or gradual, over tens of meters. Soil also varies vertically, as a function of depth. Changes in soil relating to either of these dimensions are sensitive to the influences of nature and human activity,

Collection Guidelines

The nature of soil makes it imperative that investigators properly document the exact location from which they collect soil samples. Hand-drawn or detailed commercial maps best illustrate specimen collection sites, as well as their spatial relationships.

Questioned samples taken from the ground surface, such as those taken from the tread pattern of a shoe. should be compared to known specimens collected from like places. Further, because time governs the factors that affect soil formation, timeliness in evidence collection is important.

To ensure that examiners possess an adequate representation of soil variability, investigators should collect a sufficient number of known soil specimens at crime scenes and from surrounding areas. Establishing the uniqueness of the soil at a particular location to the exclusion of others greatly strengthens the association between specimens.

Of course, the available amount of suitable soil can limit the significance of the comparison. While in most cases, investigators cannot control the amount of questioned soil available for comparison, they do have substantial control over the number of known specimens collected.

In most cases, a 35mm film canister of soil from each location is sufficient for comparison. The nature of the crime scene and the investigation generally dictate the number of samples needed.

All samples should be packaged dry, sealed, and properly labeled. Investigators must allow moist soil samples to air dry overnight at room temperature before packaging. Overlooking this step has resulted in the receipt of some rather exotic "terrariums" within samples, Plant nutritional demands can also alter soil characteristics, and consequently, undermine the effort involved and the value of the soil comparison.

In addition, investigators should not overlook the collection of alibi soil samples. They should collect these alibi samples from any area that suspects could claim as the source of the questioned soil. A suspect may contend, for example, that soil recovered from the shovel used to dig a victim's rave actually came from a garden. As with the New York case, if forensic examiners can identify dissimilarities between the soil found on a shovel and that of the suspect's garden or yard, they can eliminate the garden or yard as possible sources.

FORENSIC SOIL EXAMINATION

When soil samples and related items are forwarded to the FBI Laboratory, qualified examiners conduct a forensic soil examination. This examination compares two or more specimens to determine if the soil can be linked by demonstrating a common origin.

Laboratory personnel perform the examination by comparing the color, texture. and composition of the soil samples. Because these characteristics result from locality-dependent factors and are sensitive to a variety of influences, differences in the characteristics tend to disassociate two soil samples. Therefore, proper documentation of an adequate number of samples greatly increases the likelihood of associating soils that share a common origin. This, in turn, can provide crucial forensic evidence to associate--or disassociate--suspects with particular crime scenes.

CONCLUSION

While forensic soil examinations can yield important information concerning crimes, successful results depend on proper evidence collection and handling by case investigators. By understanding the vulnerability of earthen materials to contamination, and by following appropriate packaging procedures, investigators can preserve the potential forensic value of soil-related evidence.

Crime Data

Law Enforcement Officers Slain

During 1992, 59 law enforcement officers were killed feloniously in the line of duty, according to preliminary figures released by the FBI's Uniform Crime Reporting Program. This represents the lowest annual total of officer deaths recorded in the past 20 years.

As in previous years, firearms continued to be the weapon most used in the slayings. During 1992, handguns were used in 40 of the murders, rifles in 9, and shotguns in 2. In addition, one officer was killed with a knife, one was killed by a bomb explosion, two were beaten with blunt objects, and four were intentionally struck by vehicles.

Twenty-five officers were slain during arrest situations, including 9 while preventing robberies or apprehending robbery suspects, 5 while apprehending burglary suspects, 3 while involved in drug-related situations, and 8 while attempting arrests for other crimes. Eleven officers were answering disturbance calls when slain, 9 were enforcing traffic calls, 7 were investigating suspicious persons or circumstances, 4 were ambushed, 2 were handling mentally deranged persons, and 1 was handling a prisoner.

Nineteen officers were wearing body armor at the time of their deaths, and 3 were slain with their own weapons. Law enforcement agencies have cleared 54 of the 59 slayings.

Geographically, the Southern State recorded 27 officer slayings; the Western States, 13; the Northeastern States, 8; the Midwestern States, 6, and Puerto Rico, 5. An additional 63 officers lost their lives due to accidents that occurred while performing their duties.

COPYRIGHT 1993 Federal Bureau of Investigation
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