Misconceptions of Fingerprints as Courtroom Evidence


On March 11, 2004, terrorists coordinated a bombing of the Madrid commuter train system that killed 191 people and injured around 2,000. Two latent fingerprints were discovered on a bag of detonators and explosives. When Spanish National Police (SNP) could not find any potential matches in their records, they sent the prints over to other law enforcement agencies.

One of the prints returned 20 matches in the FBI database, and Brandon Mayfield was among the matches. Mayfield was a former U.S. Army platoon leader who worked as an attorney specializing in child custody, divorce, and immigration law in Portland, Oregon. His prints were in the system because of his military background as well as a 1984 arrest for burglary that turned out to be a misunderstanding (the charges were dropped).

FBI Senior Fingerprint Examiner Terry Green identified Mayfield as the source of the latent print, which was verified by Supervisory Fingerprint Specialist Michael Wieners, Unit Chief of the Latent Print Unit and fingerprint examiner John T. Massey, a retired
FBI fingerprint examiner with over thirty years of experience.

The fingerprint identification, which Green considered “to be a 100% identification” of Mayfield, was support to conduct a full investigation including 24-hour surveillance and physical searches. During background checks, the FBI learned that Mayfield had become a Muslim after meeting his wife, an Egyptian immigrant. He had also represented a member of the Portland Seven, a group of men who tried to travel to Afghanistan to fight for al Qaeda and the Taliban against U.S. and coalition forces, in a child custody case.

Mayfield maintained he was innocent. His passport has expired, and the last record of international travel was during his military service in 1994. However, the FBI learned that Mayfield had taken flying lessons in the past and found Internet searches about flights to Spain (as a result of his daughter doing a school project) – more coincidences that had them convinced they had their man.

By mid-April, Spanish authorities told the FBI that their own examination of Mayfield’s fingerprints did not match the fingerprints found on the bag, but the FBI doubled down and arrested Mayfield as a material witness on May 6. The FBI continued to stand by their findings until May 19 when Spanish police conclusively matched the print to the real offender, Ouhane Daoud, an Algerian national living in Spain.

Mayfield was awarded a $2 million settlement for his false imprisonment, and the FBI was forced to formally apologize. They issued a statement promising to assess procedures saying, “The FBI identification was based on an image of substandard quality, which was particularly problematic because of the remarkable number of points of similarity between Mayfield’s prints and the print details in the images submitted to the FBI. The FBI’s Latent Fingerprint Unit will be reviewing its current practices and will give consideration to adopting new guidelines for all examiners receiving latent print images when the original evidence is not included.”

A Brief History of Fingerprint Identification
Fingerprint evidence has the reputation of being “infallible.” We have been told that no two fingerprints are the same and watch as investigators on popular tv crime scene shows make positive matches with fingerprints. While facial features and other physical characteristics change throughout one’s lifetime, fingerprints are relied upon because the unique patterns of ridges and valleys are considered unchanging, growing with us but maintaining their shape from about six months gestation to the grave.

Records show the Chinese used handprints as evidence during burglary investigations as early as 206 BC. Throughout the 1600-1800s, several anatomists studied and made notes of the differences in friction ridge skin patterns. The first classification system for fingerprints was published by Sir Francis Galton in his book “Finger Prints” in 1892, and the Calcutta (now Kolkata) Anthropometric Bureau became the world’s first Fingerprint Bureau for classification of criminal records in 1897.

In 1879, French criminologist Alphonse Bertillon invented the Bertillon system of anthropometric measurement also known as Bertillonage, using 11 specific body dimensions such as such as head length and width, length of the middle finger, length of the left foot, and length of the forearm from the elbow to the tip of the middle finger to produce a detailed description of an individual and identify criminals. Bertillon calculated that the probability of two people having exactly the same 11 measurements was one in four million. The Bertillon system was the dominant criminal identification method both in the U.S. and Europe until the case of the West brothers demonstrated the reliability of the emerging science of fingerprint identification over that of the Bertillon system.

In 1903, a man named Will West was sentenced to be incarcerated at the United States Penitentiary at Leavenworth, Kansas. He was photographed and measured using the Bertillon system. Will West denied ever being held at the facility, but his measurements were nearly identical to a Bertillon card under the name of William West and the two looked very similar. An investigation found that a prisoner named William West had been imprisoned at the penitentiary since 1901 and was serving a life sentence for committing murder. Fingerprints identified the difference between the two men, and fingerprint analysis eventually displaced Bertillon’s anthropometric measurement system as the preferred criminal identification system.

Shortcomings of Fingerprints
Just like there were flaws with the Bertillon system, we are now realizing that fingerprint analysis isn’t as foolproof as we are led to believe. Sir Galton, the author of “Finger Prints,” estimated that the odds of two individual fingerprints being the same were 1 in 64 billion. Not likely odds for sure, but not impossible. Families can share elements of the same pattern. Skin loses elasticity as we age, and certain conditions can leave some people with smooth, featureless fingertips.

When it comes to criminal investigations, one of the biggest drawbacks of forensic fingerprint identification is the propensity of human error and bias, as evidenced by the Mayfield case. In 2005, Simon Cole, a criminologist at the University of California at Irvine, published a study detailing 22 known cases of fingerprint mistakes in the history of the American legal system and surmises there are many more cases of misattribution.

Studies have shown that experts can make different judgments when presented with the same evidence. A Southampton University study found that two thirds of experts, who were unknowingly given the same sets of prints twice, came to a different conclusion on the second occasion.

Furthermore, there is no established minimum or standard for how many points of similarity constitutes a match. Local practices vary, so it’s probable that fingerprint examiners in different locations can come to separate conclusions.

Unlike other forensic fields, like DNA analysis, which give a statistical probability of a match, fingerprint examiners traditionally testify that the latent print is either a 100 percent match or a 100 percent exclusion. The word “match” itself can be misleading.

Nick Petraco, assistant professor at John Jay College of Criminal Justice at the City University of New York and a forensic consultant to the NYPD, says, “Basically it’s a word that most people associate with ‘identical,’ meaning that it’s the exact same and that there’s no other alternative. So if you say in relationship to a hair examination ‘the hair matched,’ to them that means that it’s from that guy’s head or from that guy’s body.”

Consistent or concordant would be a more realistic way to phrase it. “Consistent means that they have the same properties and they could be from that location… but not necessarily,” Petraco says.

Fingerprint databases usually contain rolled fingerprints from each finger known as ”tenprints” and fingerprints from the whole hand with all the fingers extended in parallel called ”slaps.” However, no two impressions are completely the same in every detail, even when recorded by the same person immediately after each other. If fingerprint impressions are not identical in controlled situations, consider the fact that fingerprints left at crime scenes are often smudged, dirty, or partial prints.

So are fingerprints reliable as evidence or not? The true answer is we simply don’t know. The potential error rate for fingerprint identification has not been studied enough to provide a credible answer. While fingerprint analysis has no doubt been valuable in locating suspects and building criminal cases, fingerprint identification is not flawless and should be treated with greater skepticism when people’s lives are on the line.