The Supreme Court on Monday upheld a law that lets the U.S. government keep inmates behind bars who have served their time but are deemed "sexually dangerous."By a 7-2 vote, the justices rejected a claim that Congress had exceeded its authority in passing the 2006 federal law.

Writing for the majority, Justice Stephen Breyer said Congress' ability to control the release of dangerous prisoners stems from its expansive power under the Constitution to enact laws governing prisons. He added that Congress has long been involved in mental health care for federal prisoners, including their civil commitment.

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Most broadly, the court said the power to confine inmates whose sentences have ended arises from Congress' constitutional authority to make laws "necessary and proper" to carry out its powers.

Breyer noted that the law has been used against a small fraction of prisoners. He referred to a statement by Solicitor General Elena Kagan at January arguments that the law has been used on 105 individuals, out of more than 188,000 inmates.

Kagan, who is President Obama's nominee to fill the court's vacancy when Justice John Paul Stevens retires this summer, had argued that the federal government was trying to fill a gap left by the states. "The federal government has mentally ill, seriously dangerous persons in its custody," Kagan had argued. "It knows that those persons, if released, will commit serious sexual offenses, and it knows, too, that states are not often in a position to deal with those dangers."

Kagan was appealing a decision by the U.S. Court of Appeals for the 4th Circuit, based in Richmond, Va., that said Congress had overstepped its power in the law authorizing civil commitment of certain prisoners.

Justices Antonin Scalia and Clarence Thomas dissented. Thomas, writing for the pair, said the majority decision allows the federal government to tread on the states and "endorses the precise abuse of power (the Constitution) is designed to prevent."

Five prisoners had brought the case. Lead challenger Graydon Comstock had six days remaining before he was to finish a sentence in November 2006 for possession of child pornography when he was certified by then-attorney general Alberto Gonzales as "sexually dangerous." Since then, Comstock has been confined at a federal prison in North Carolina.