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 lab in an unrelated case. He was freed. Another man ultimately was arrested in the killing but died before trial.

DNA has proved itself by far the most effective and reliable forensic science. Over the last two decades, it has solved crimes once thought unsolvable, brought elusive murderers and rapists to justice years after their misdeeds and exonerated the innocent. In courtrooms and in the popular imagination, it is often seen as unassailable.

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

* Although best known for clearing the wrongfully convicted, DNA evidence has on occasion linked innocent people to crimes. In the lab, it can be contaminated or mislabeled; samples can be switched. In the courtroom, its significance has often been overstated by lawyers or misunderstood by jurors.

* 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. Some crime labs have huge backlogs of untested evidence, including thousands of rape evidence kits. In some cases, criminals who could have been caught have offended again.

* Debates have flared over civil rights and privacy, presaging possible constitutional challenges to DNA collection and storage. Critics object, for instance, to storing DNA from people arrested but not convicted of crimes and from suspected illegal immigrants.

In Britain, which has the world's most aggressive approach to forensic DNA, a legal backlash has already begun. The European High Court of Human Rights ruled this month that the country's indefinite storage of DNA from people merely arrested for crimes violated privacy rights. Britain has until March to submit plans for destroying samples or to make a case for keeping them.

In the U.S., authorities are plunging ahead with a dramatic databank expansion.

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

California's database is expected to swell by about 300,000 DNA profiles next year, bringing the total to 1.4 million.

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

Nothing to fear

When the California law, Proposition 69, passed, it was widely believed that the innocent had nothing to fear from having their genetic profiles in a database, said UC Irvine criminology professor William Thompson, considered the U.S. leading authority on DNA laboratory error.

Now, he said, "when you look at all the errors that have come to light around the world -- and we're only finding the tip of the iceberg -- it really raises concerns about how many people you want to have in a database. There are certainly doubts in my mind whether I would want to be in one."

Through the California Public Records Act, The Times obtained documents from five state-run and three county forensic labs reporting scores of laboratory errors or "unexpected" resultsover a five-year period ending in 2007. Labs must track these outcomes and keep them on file under state and federal rules.

Thompson, who reviewed the records for The Times, said that "on a regular basis, laboratory personnel make mistakes that could lead to false identifications" of suspects.