I want to discuss something most of us would have once considered to be as unlikely as the confluence of weather systems that caused this tragic storm...a latent fingerprint matched to two different men.
On March 11, 2004, a series of 10 bombs exploded on commuter trains in Madrid, Spain, resulting in nearly 200 deaths. A fingerprint found on a bag of bomb-making equipment in a vehicle became a crucial piece of evidence.
The FBI entered the print into its database and 20 possible matches were generated. Each shared a minimum of seven unique traits with the print in evidence. To make a "positive ID," there would have to be 12 matching traits. One set of fingerprints on file for a lawyer in Portland, Oregon, shared 15 traits with the print in evidence. He must be the bomber, right?
At the time, a U.S. counter-terrorism official told Newsweek it was an "absolute incontrovertible match."
Later, Spanish authorities found the prints of a second man, a known terrorist, that also "matched" the sample in evidence. Parts of those fingerprints were so similar that both men were POSITIVELY IDENTIFIED as the bomb maker. It turns out that processing and interpreting fingerprint evidence is not an exact science, yet. It is valuable, but it is not absolute.
The National Academy of Sciences studied the discipline. In July 2009, they found there is inadequate "scientific rigor" in the methods and procedures.
This case and others are highlighted in the NOVA program "Forensics on Trial" which aired this month. (It can be viewed online here). It points out human errors in this case. It also introduces CTF impressions, a new technology that replicates the topographic features of fingerprints without altering them. Its possible field use is under review.
More sophisticated methods would be welcome, but we must deal with examiner bias and set reasonable standards to further improve our justice system.
See these sources for more details:
NOVA: Forensics on Trial, Aired on PBS, October 17, 2012
FBI is Cleared of Misconduct in Jailing of Oregon Man, NY Times, January 6, 2006
U.S. Settles Suit Filed by Ore. Lawyer, Washington Post, November 30, 2006
Badly Fragmented Forensic Science System Needs Overhaul, National Academy of Sciences, 2009