In 2004, a New Jersey prosecutor announced that DNA had solved the mystery of who killed Jane Durrua, an eighth-grader who was raped, beaten and strangled 36 years earlier.

“Through DNA, we put a face to the killer of Jane Durrua, and that face belongs to Jerry Bellamy,'' prosecutor John Kaye said. The killer, however, turned out to be someone else.

Two years after Bellamy's arrest, investigators discovered that evidence from the murder scene had been contaminated by DNA from Bellamy, whose genetic sample was being tested at the same laboratory in an unrelated case.

He was freed. Another man ultimately was arrested but died before trial.

DNA has proved itself by far the most effective and reliable forensic science.

Over the past two decades, it has solved crimes once thought unsolvable, brought elusive murderers and rapists to justice years after their misdeeds and exonerated innocent people.

In courtrooms and in the popular imagination, it often is seen as unassailable.

But as the United States rushes to take advantage of DNA's powers, it is becoming clear that genetic sleuthing has significant limitations:

— In the laboratory, DNA evidence can be contaminated or mislabelled; samples can be switched. In the courtroom, its significance has been overstated.

— The rush to collect DNA and build databases has in some cases overwhelmed the ability of investigators to process the evidence and follow up on promising leads.

— Debates have flared over civil rights and privacy, presaging possible constitutional challenges to DNA collection and storage.

Adding to the base

In Britain, which has the most aggressive approach to forensic DNA, a legal backlash has occurred.

The European High Court of Human Rights recently ruled that the country's indefinite storage of DNA from arrested people violated privacy rights. In the US, authorities are plunging ahead with a dramatic databank expansion.

A California law passed in 2004 will permit authorities to store DNA from anyone arrested on suspicion of felonies and serious misdemeanours, even if they are not ultimately convicted.

The FBI's national database, which contains 6.4 million profiles, is projected to add about 1.3 million annually from federal arrestees and illegal immigrants alone.

When the California law, Proposition 69, was passed, it widely was believed that the innocent had nothing to fear, said William Thompson, a criminology professor at the University of California, Irvine, who is considered the leading US authority on DNA laboratory error.

Now, he said, “when you look at all the errors that have come to light around the world it raises concerns about how many people you want to have in a database. There are doubts in my mind whether I would want to be in one.''

The leading cause of false DNA database matches is cross-contamination, Thompson said.

Wrongful incriminations from DNA evidence have pierced the science's image of infallibility.

When Alan Nelson, father of a woman who had been murdered with her daughter in Australia, learnt earlier this year that the wrong man had been arrested because of sample contamination, he was incredulous: “I thought the DNA was 100 per cent perfect,'' Nelson told an Australian newspaper.

Even the most scrupulous analyst can err when interpreting complex DNA “mixtures'' — samples that contain DNA from more than one person — which turn up more frequently as laboratories use more sensitive tests.

In a 2005 study, US laboratories that analysed the same DNA mixture reported different statistics on the rarity of the genetic profiles of the contributors.

Estimates of how often the DNA profiles occurred in the general population ranged from 1 in 400,000 people to 1 in 1,000,000,000,000,000.

In the courtroom, some leaders in the field say, the overwhelming numbers presented to jurors convey a near-certainty that the science simply does not justify.

Civil libertarians and other critics, meanwhile, fear that pursuit of DNA's scientific potential risks trampling on the rights of the innocent.

“The American Civil Liberties Union (ACLU) opposed these databases from the beginning because we predicted this exactly,'' said Tania Simoncelli, science adviser to the ACLU.

In Britain, such concerns have brought what Bob Green, a forensic science director in Britain's Home Office, calls a “push back''.

Last year, Britain's Nuffield Council on Bioethics complained that too many people were in the British database for petty crimes they committed as children or for arrests that did not produce convictions.

Alec Jeffreys, the British geneticist whose discovery eventually led to DNA profiling, is dismayed by the number of juveniles and innocent people in the database. He figured DNA would be a “last resort'' crime-fighting tool.

“I couldn't have been more wrong,'' he said.