In old-fashioned murder mysteries, the killer's sweaty palms always give him away. The intuitive detective -- attentive, adept at reading human nature -- quickly recognizes them as a sure sign of guilt.
Now, investigators say they require not intuition but DNA. They need only a drop of the sweat.
"When a person leaves any DNA at his crime scene, whether it's a drop of blood, saliva or perspiration, he's left us his calling card," says Paul Ferrara, a noted DNA researcher and head of Virginia's state crime laboratory, the Division of Forensic Science. "Just wearing a pair of gloves doesn't throw off the trail anymore."
Forensic science used to seem a simpler, less controversial discipline, as presented in the first manual on the subject, "The Washing Away of Wrongs," written in China in 1248. That book offered simple conclusions: Water in the lungs indicated drowning; pressure marks on the throat hinted at strangulation.
The field has advanced to the point that investigators can help identify a murderer by the insects on the corpse or a drop of saliva left behind by the killer. Fibers from a killer's gloves can be the breakthrough in a case, if there are traces of sweat on them. According to Ferrara, an important clue in a recent Virginia kidnapping case came from the perspiration on the band of a hat the abductor left behind.
"We surprise a lot of people with it," says Ferrara, describing the DNA analysis known as polymerase chain reaction, or PCR, which enables scientists to clone minute samples of DNA. "We've gotten DNA based on saliva from cigarette butts, saliva from the backs of envelopes, tissue from under the victim's fingernails, and from a portion of a hair follicle."
He and other forensic specialists have begun playing a greater role in crime investigations, based on sophisticated, sometimes controversial, methods of identifying criminals at their most basic level -- their genetic code.
Serologists used the saliva from an envelope flap of an tTC anonymous letter to link terrorist Nidal Ayyad to the 1993 World Trade Center bombing; Ayyad and others were later convicted. The FBI is also trying to link DNA from saliva on a postage stamp to the person accused of being the Unabomber, Theodore J. Kaczynski.
An advantage of PCR over older methods of DNA fingerprinting is that technicians can now analyze far smaller samples taken from bodily fluids.
Cellular material in saliva and sweat often leaves the critical clues, says Toby L. Wolson, a DNA analyst and blood stain expert with the Metro-Dade police department in Miami.
"I have extracted DNA from a drop of blood smaller than the head of a pin," says Wolson, who heads a group called the International Association of Bloodstain Pattern Analysis. The group is made up of crime lab technicians and scientists from around the country who interpret the patterns of blood in violent crimes.
While the acceptability and reliability of DNA testing continues to be controversial, as in O. J. Simpson's trial in the slayings of his former wife and her friend, genetic biology has found a permanent place in the U.S. justice system.
Forensic experts with the FBI laboratory in Washington are developing a national database, the Combined DNA Index System, to allow police to link DNA from crime scenes with state databases of convicted sex offenders.
As of June, Maryland and 41 other states had passed legislation requiring convicted sex offenders to provide biological samples. Some states require all felons in the prison system to submit samples; others are considering collecting DNA from even minor criminals.
The "DNA data bank" has been heralded by law enforcement officials as the greatest change in criminal justice since fingerprint technology became available about a century ago. But critics fear the technology could turn government and big business into keepers of a DNA warehouse.
"DNA profiling poses a special risk of invasion of privacy," the National Research Council of scientists reported in 1992, shortly after the databases were proposed. "Such information could lead to discrimination by insurance companies, employers or others against people with particular traits."
Lawrence Kobilinsky, a professor of forensics at the John Jay College of Criminal Justice in New York, says one of the major questions in DNA technology is the thoroughness of the testing. Most of the crime lab work in the United States depends on DNA testing kits prepared and sold by Perkin-Elmer Corp.
As with any courtroom evidence, results should be checked and re-checked, Kobilinsky says.
"I believe strongly in DNA, but I also think that if you're going to put somebody to death based on DNA analysis, you'd want to have your tests optimized," he says. "We owe it to a defendant to do the best science possible, and we're not always doing that."