WASHINGTON – The Supreme Court said Thursday that DNA possesses a unique ability to free the innocent and convict the guilty, but the justices nonetheless ruled that prisoners do not have a constitutional right to demand DNA testing of evidence that remains in police files.

In a 5-4 ruling, the court’s conservative bloc agreed to allow states to decide rules for new testing of old crime samples.

Already, 47 states and the federal government have enacted laws or rules that allow prisoners under some circumstances to obtain DNA tests, the high court said.

Chief Justice John Roberts said the majority saw no need for “a freestanding and far-reaching constitutional right of access to this type of evidence.” Upholding such a new right “would take the development of rules and procedures in this area of out of the hands of legislatures and state courts shaping policy in a focused manner and turn it over to federal courts,” he said.

While Roberts stressed the virtues of judicial restraint, the dissenters said the court was abdicating its duty to seek justice.

Alaska does not give prisoners a right to obtain DNA testing, and William Osborne, a convicted rapist, belatedly sought testing of a semen sample. He and another man were accused of abducting a prostitute near Anchorage, beating her and leaving her nearly dead in the snow. She survived and identified Osborne as her attacker.

His lawyer did not seek DNA testing during his trial, but he sued to obtain the tests after his conviction. Osborne even offered to pay for the test.

Justice John Paul Stevens, in dissent, said Alaska has the evidence that “will conclusively establish” whether Osborne committed the rape.

“If he did, justice has been served by his conviction and sentence,” Stevens wrote. “If not, Osborne has needlessly spent decades behind bars while the true culprit has not been brought to justice.”

Stevens said the prisoner in this situation has a right to “test the evidence at his own expense and to thereby ascertain the truth once and for all.”

Justices Ruth Bader Ginsburg, David Souter and Stephen Breyer joined in dissent.

Besides Alaska, only Massachusetts and Oklahoma have not decreed by law that at least some convicts can obtain DNA testing.