April 9, 2003
A: FROM MENTOR ROSE CLARK IN PA
I am not an expert at fingerprints but do teach forensic science. If
you think about fingerprint, you know that they are cause by grease from
you hands when you contact another object. That grease will be smeared
and covered up over time and will not be distinguishable. When you are
looking for fingerprints you only want the fresh fingerprints which will
be the most obvious. You will also want to look at specific items in
the room which might have been used recently. You do not want to dust
the whole room because you will get too much information to analyze. We
had a police office in to speak to our students last semester and he
emphasized that when collecting evidence you need to be very selective.
You can not analyze every fiber in the room (or fingerprint).
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A: FROM MENTOR CHRISTINE KUTA IN MA

Actually, the FBI (and the police in general) typically do find the
fingerprints of lots of people including the suspect but also of other people
who have been in the room. The way the police narrow down the fingerprints
to the suspect is they take the fingerprints of all the people known to have
been there and then eliminate from consideration the fingerprints of all
people who are not suspects in whatever crime was committed. The
fingerprints that are left may belong to the criminal or they may belong to
innocent people that the police do not know about. Fingerprint detection and
matching is actually an inexact science, not nearly as easy to interpret or
as clear an indication of guilt as in the detective novels or on TV.
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A: FROM MENTOR AMY MCMILLAN IN NY
Hi Melisange, I don't know very much about fingerprints but it interests me
so I looked on the web for some information. This is a very interesting
site - everything you always wanted to know about fingerprints...
http://www.fingerprints.demon.nl/ some of the information at this website
explained that fingerprints do have a time line. Fingerprints are usually
left by oils that are naturally on the skin or by an amino acid residue. (or
of course, by indentations in something soft or by other materials, such as
blood, on the fingers). How long they last depends on what surface they
were left on. Apparently, prints can be retrieved from paper after several
years! How long prints can be detected also depends on the environmental
conditions and on the person leaving them. Some people have dry, not very
greasy fingers and so their prints are less noticeable and last for a
shorter time. Others have greasy fingers... also, sun and wind dry prints
out and rain can wash them away. The website says prints usually last only a
few days to a week...
In answer to your question about how the FBI agents find the fingerprints of
just the targeted (guilty?) person in aroom but not everyone else... well,
they do find fingerprints from anyone else who left them in a room within
the last week or so. The investigative part of this work is to figure out
whose fingerprints matter... My guess is that a good investigator has a good
feel for the age of fingerprints - that helps when the investigator
understands the crime time line. The rest of them would need to be
eliminated based on other evidence - like alibis, or many other factors that
would rule certain people innocent of the crime...
I am sure other people know much more about this than I! Hope someone who
is an expert in this field will correct me or add other interesting details!
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A: FROM MENTOR JOAN LUSK IN RI
You're right, they _do_ detect all the other fingerprints. The FBI lab has to compare whatever prints the agents have found with prints from all the people known to have been at the site and the suspects who might have been at the sight, to see if there are any matches. Sometimes you'll see a news report that mentions that the only prints on a stolen, recovered gun were its rightful owner's, for example.

Here's a web page describing how they analyze fingerprints; I didn't notice any mention of how long a latent print will last if undisturbed.
http://www.c3.lanl.gov/~brislawn/FBI/FBI.html
At the FBI site there is a page on employment opportunities, including internships for college students.
http://www.fbi.gov/employment/employ.htm
But at part of the FBI site designed for kids, the Bureau describes fingerprinting, DNA testing and polygraph testing (lie detectors) as if they were equally accurate. Lie detectors have been pretty thoroughly discredited; people can learn to be good liars and fool them. In my personal opinion (as a biochemist and therefore biased in favor of molecular methods), DNA testing has the ability to be more accurate than fingerprinting - one can always compare more sites in the DNA but suspects have only ten fingers. (Either method of course can fail if the samples are not good or if someone plants false evidence; and a DNA sample may be too small to test for more and more sites.)

It's interesting that the accuracy of DNA testing has aroused controversy while evidence from fingerprints and even lie detectors has long been admissible in court. That is probably because everyone feels they understand fingerprints - just look at your fingers - and everyone knows what it feels like to tell a lie, but most people don't know much about DNA. And we all think we are good, observant witnesses, despite the fact that experiments show that people are not good at noticing and remembering events and people accurately.