Unit 3: Forensic Science - Physical Evidence

Subpoena

A formalized demand that the witness will provide evidence or testimony in court. Although they are not always necessary, they are an important tool for investigators and attorneys during a criminal case.

Photo Array

A series of unmarked photos showing different people who physically resemble the suspect, mixed with a photo of the suspect. Sometimes a witness can pick out the suspect from the array. Police may also show witnesses photos of multiple suspects or a single picture of the suspect to confirm identity.

Documentary Evidence

Any type of written or recorded evidence, such as a recording of a telephone conversation or a video recording of a crime. The recording is typically authenticated by an expert to show that it hasn't been doctored or manipulated in any way.

How to take impressions

As a first step, investigators take photographs of the impression. They put a ruler in the photos for scale and to show the size of the impression. Good photos pick up the details. As with objects bearing tool marks, forensic scientists may collect the whole object that the impression is in, but it isn't always possible, particularly when the impression is left in the dirt by a tire or shoe. In this case, a cast is made. The forensic scientist places a frame around the impression to hold the casting material as it's poured.

Casting Material

Casting material may be silicone rubber or dental stone. In the past, Plaster of Paris was often used to make casts, but the substance has been widely discontinued since it crumbles more easily than other materials. Casts need time to sit and cure before they can be analyzed, curing usually takes up to 24 hours.

Impressions

Come from shoes, tires, or other objects that may be found. It can be deep ruts in the ground from a car going through mud or fine, thin impressions of a shoe on a dusty floor. As with tool marks, they can give either individual or class characteristics depending on the impression and what made it. It may show class characteristics, for example, investigators might be able to identify the type of tire or a particular brand of shoe that made the impression. Sometimes impressions may show individual characteristics too, it might come from a source that has unique features do to wear.

Testimony

Evidence given by witnesses during a trial. It might come from someone who witnessed the crime, or from someone who testifies as a character witness for the suspect. Witnesses who saw the crime take place can potentially provide a special type of evidence: eyewitness identification. If investigators don't have a suspect or if they have multiple suspects, they can use eyewitness identification to narrow down the field. Police may obtain a general description of the suspect and create a sketch from the description. Or they may ask the witness to identify the suspect from a photo array.

Demonstrative Evidence

Evidence that is used to help recreate or illustrate the crime during a trial and show where items were at the scene.

How do investigators sort through important evidence and analyze it once it's in the crime lab?

First, forensic scientists take measurements to help identify the characteristics of the evidence, including chemical makeup.

What does these qualifications mean to the forensic scientists?

First, the test has the show that results are reproducible or reliable. The test should produce the same result multiple times for the same samples. If a test is shows that a soil sample has a high level of a certain mineral, the test would be reproducible if this result appeared in more than one test. Second, a test must be sensitive in sense that it identifies the unique characteristics of the sample. For example: a DNA test can identify people based on their unique DNA. Finally, tests are specific if they identify particular substances. A test would be specific if it identified the presence of a particular drug, like cocaine, with no other drugs giving the same results. Forensic scientists must choose the right test in order to get the most information out of evidence.

Soil

For forensic scientist soil is more than dirt. Soil is thought of as surface material from the earth, both natural and artificial. To a forensic scientist, soil includes not only dirt, but also rock, animal material, and vegetation lying near the surface of the ground. Soil may also include pieces of glass, fragments of brick or stone, and pieces of asphalt. The combination of materials in soil may link a suspect to the crime scene if the soil is transferred to the suspects shoes or clothing. Forensic scientists compare soil samples to see if there is a match that links the suspect to the crime. Many soils can be distinguished by color and texture. While visual differences are easy to spot, it's important to note that soil will look darker when wet. For this reason, color comparison should be done when soil samples are dry. Microscopic analysis can reveal the presence or absence of vegetation or animal material. Artificial materials, like glass particles, are also easily seen under a microscope.

Types of Evidence

Four different types of evidence that can be part of a criminal investigation.

Identifying Physical Evidence

Generally there are more objects within a crime scene that investigators can collect and analyze. Objects that might be physical evidence includes, everything from glass particles, to fibers from clothing, to blood or other bodily fluids. Many items might also be used as weapons, making them evidence as well.

Admissible Evidence

Important to collect evidence properly so that the evidence is admissible. This goes for both physical evidence and non-physical evidence, like testimony. Investigators must be careful to follow correct legal procedures for obtaining evidence from witnesses. Must also tread carefully in suspect interviews.

How do forensic scientists decide what tests to use when testing evidence?

In many cases, a number of tests may be done on the same piece of evidence. But it depends on the test they decide, forensic scientists consider a number of different factors when deciding on tests. For example: they consider the quantity and quality of the sample. Some tests may require a certain amount of of a material to produce results. These tests may not be used if only trace evidence of a particular substance is available.

Physical Evidence

Includes tangible objects such as weapons, trace evidence like hair, blood, or fibers, as well as fingerprints. These items are real, direct evidence, of the crime and the individuals involved.

A Forensic Geologist

May use a high-powered microscope to examine soil samples for rocks and minerals and use these findings to determine whether samples are similar. Due to the wide range of materials found in soul, they can finds many points of comparison for two soil samples. The more similarities between these points, the more likely it is that the two samples come from the same place. However, a range of soil similarities must be taken into consideration during examination. If the soil is uniform for a mile around the crime scene, a sample of this soil is less useful than it would be if the soil type was unique to the scene or the a particular spot within a scene. Rare minerals and rocks or artificial debris can narrow down possible soil matches. Soil samples typically collected from various spots at the scene within a 100-yard radius.

What if the samples share the same characteristics?

Might be tempted to conclude that the evidence 'matches', but forensic scientists look at the probability that the two samples come from the same source.

Collection of Soil

Only surface samples are taken, since the soil may show misleading differences if a deeper sample is taken. Each sample is generally about one or two tablespoons in volume. During collection, each sample should be clearly labeled with the location it was taken from. Forensic scientists take great care in collecting and preserving soil samples. If soil is found on the bottom of a shoe, the soil is left on the shoe and the entire shoe is taken to the crime lab. An item with soil must be stored in such a way that dry pieces of dirt that fall off are retained.

Probability

The frequency of an event. Using this, forensic scientists conclude how likely it is that two sources would share the same characteristics. Evidence testing follows strict guidelines. In order to be valid, a particular test has be to reliable (or reproducible), sensitive, and specific.

Identification

The process of determining the chemical or physical characteristics of a piece of evidence with as much certainty as possible. Example: A powder may have been collected at a crime scene. Forensic scientists use tests to determine whether the powder is a drug, explosive material, or a fool like powdered sugar. It may also determine if a hair sample is from a human or an animal.

Comparision

The process of testing a suspect sample against a known sample, in order to prove whether or not they share a common origin. Example:A sample of blood from a crime scene may be compared to a sample of blood from a victim, suspect, and possibly other individuals who lived or worked at the crime scene.

Soil found on a suspect

The soil will be collected, along with soil from any alibi locations.

Electrostatic Dust Print Lifter

When an impression is found in dust, forensic scientists may use this. With this method large negative charge is applied to a plastic-coated metal sheet. The dust print will appear as a mirror image on the plastic coated sheet. This technique may be used on rough surfaces too. For smooth surfaces, clear tape may be used to lift the impression.

Uncooperative Witnesses

Working with witnesses isn't always easy. Some may be uncooperative and withhold important evidence from investigators, and others may refuse to testify at a trial. In these cases, law enforcement or attorneys can issue a subpoena.