Fingerprints were first successfully used in 1892, when Argentine detective Juan Vucetich used them to solve a double murder. A decade later, fingerprinting was introduced in the United States by Henry DeForrest at the New York Civil Service Commission. Today the FBI has more than 250 million sets of computerized fingerprint records in its Integrated Automated Fingerprint Identification System.

Fingerprinting is generally one of the first things that is done with criminal suspects in the United States after they've been read their Miranda rights. It is long settled law that fingerprinting does not violate the Fifth Amendment's guarantee against compelled self-incrimination or the Fourth Amendment's prohibition of unreasonable searches and seizures. Indeed, much non-testimonial evidence does not fall under those constitutional protections, including the analysis of blood and breath for alcohol. Samples of semen, hair, and other tissues may be taken without a suspect's consent.

Now comes DNA testing. In recent years, DNA testing has proved a very useful tool in solving crimes. It has also been very useful in exonerating people wrongly convicted of crimes they did not commit. All 50 states have already established or have begun to establish DNA crime data banks. In 1998 the FBI set up the National DNA Index System to link the various state data banks. At first the circumstances under which the police were permitted to collect DNA samples were highly restricted, but legislatures are beginning to relax those restrictions.

Virginia, one of the first states to set up a DNA crime data bank, initially collected samples only from people convicted of serious felonies. Virginia's Division of Forensic Sciences used the data bank to solve 308 crimes in 2001 and 70 so far this year. Now the Virginia legislature wants to expand dramatically the pool of people subject to DNA testing. Both houses of the legislature have just passed bills under which the police could collect DNA samples from people arrested for, but not convicted of, violent crimes. Those DNA samples would be used not just to solve the crime for which the suspect was arrested but also to match DNA samples found at the scenes of other crimes.

The legislators and police argue that this expansion of DNA testing simply builds on a century's experience with ordinary fingerprinting. After all, obtaining a DNA sample with a cheek swab is not much more invasive than staining a suspect's fingers with ink, and it's a lot less invasive than alcohol blood testing or semen collection. According to this view, DNA testing is just another, perhaps more effective way to establish a suspect's identity and presence at a crime scene.

Critics worry that DNA testing has the potential to do more than that. In testimony before Congress, Barry Steinhardt, associate director of the American Civil Liberties Union, has argued that DNA data banks "should not house the genetic profiles of persons who have never been convicted of a crime. To do so is to equate arrest with guilt and to empower police officers, rather than judges and juries, with the power to force persons to provide the state with evidence that harbors many of their most intimate secrets and those of their blood relatives."

The data currently collected through DNA crime testing can only identify or rule out a suspect, since the material tested is from parts of the genome that do not include any genes. In other words, the tests are designed so they cannot tell anything about, say, genetic diseases or paternity. But DNA samples are usually retained by the police, and in the future they could be used for purposes other than identification: genetic testing for diseases or, more chillingly, research aimed at determining if there are genes that predispose certain groups to criminal behavior.

Such concerns could be greatly alleviated if the samples were destroyed once the DNA fingerprints had been determined and digitized. But some law enforcement officials don't want to destroy the samples because better and cheaper identification tests may come along. If the old samples were still in storage, police could simply retest them rather than having to start all over to build a new DNA data bank.

Law enforcement officials also argue that statutes can bar anyone but the police from using DNA fingerprints and samples. But history suggests that such a line would not hold for long. Consider two cases: Social Security numbers and ordinary fingerprinting.

The 1935 statute that created the Social Security system said the numbers would be used only for identification in that program. Today Social Security numbers are a ubiquitous form of identification, required by an ever-growing roster of federal and state agencies.

Likewise, fingerprinting is not just for law enforcement anymore. A Pennsylvania school lunch program now requires students to identify themselves with fingerprints each time they buy lunch. Even private organizations such as car rental companies are requiring fingerprints, while states such as Georgia are incorporating fingerprints into their driver's licenses.

DNA testing for crime detection is too useful to be abandoned. The real question is how much more DNA testing Americans will accept in the future. The precedents, legal and social, are not reassuring.