The US Supreme Court on Friday agreed to take up a case examining whether the Fourth Amendment permits police to collect and analyze a persons DNA at the time of arrest or whether they have to wait until after the suspect has been convicted to take a DNA sample.

The case raises the thorny issue of when the government is allowed to collect a DNA sample from an individual and store it in a national database.

DNA information has become a major crime-fighting tool, providing a significantly more accurate method of identifying individuals than the subjective art of fingerprint comparisons. DNA data have helped police solve crimes and have also helped defense lawyers identify and correct wrongful convictions.

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But unlike fingerprints, DNA material can reveal far more about an individual, including details that most people consider highly personal and private.

Nonetheless, law enforcement officials insist that they are interested in using DNA technology only for identification, as a kind of enhanced form of fingerprinting.

The issue arises in the case of Alonzo King, who was arrested in southern Maryland in 2009 on an assault charge.

As part of his processing by police, officers used a swab to collect a sample of his saliva from his cheek. The sample yielded an identifying DNA profile which was fed into a national database.

Mr. Kings DNA profile was found to match the DNA profile of the person who robbed and raped a woman in the same area of southern Maryland in 2003.

Armed with that information, authorities applied for a search warrant and obtained a second swab of Kings cheek. The second sample also matched the DNA evidence from the 2003 rape. King was charged with the rape and robbery.

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Can police collect DNA before conviction? Supreme Court to hear case