WASHINGTON - Fleeing from the police in a car can trigger a mandatory 15-year term in federal prison for a repeat criminal who is carrying a gun, the Supreme Court ruled Thursday.

Splitting 6-3, it said that "vehicular flight" counted as a "violent felony" under the Armed Career Criminal Act, triggering the mandatory term if it is a third offense.

Speeding away from the police "presents a serious potential risk of physical injury to another," Justice Anthony M. Kennedy wrote for the majority in Sykes v. United States. "It is a provocative and dangerous act that dares, and in a typical case requires, the officer to give chase."

Congress adopted the law in 1984 as an early version of a "three strikes and you're out" measure. Gun-carrying criminals with three violent felonies on their record would draw at least 15 years in prison. But the law did not carefully define which crimes counted, except to cite burglary, arson, extortion, and other acts involving force or a risk of serious injury.

The justices, with some reluctance, have taken on the task of deciding what Congress meant. They said in 2007 that attempted burglary counted, because it could lead to a violent confrontation. A year later, they said that driving under the influence did not qualify because it was not an aggressively violent act.

Marcus Sykes appealed the issue after he was sentenced to 15 years in prison for two robberies in Indianapolis, of a purse and a wristwatch. After the second incident, in 2008, an officer spotted Sykes as he tried to walk away and toss his gun aside. He pleaded guilty to gun possession and robbery.

At his sentencing hearing, prosecutors showed he had been convicted in 2002 for trying to flee police in Lawrence, Ind. He was driving a car with no headlights and tried to drive away when an officer flashed his emergency lights. He was stopped and arrested.

The justices decided Sykes' third offense counted as a "violent felony," and they upheld his 15-year prison term.

But the ruling split the justices in an unusual way. Joining Kennedy in the majority were Chief Justice John G. Roberts Jr. and Justices Stephen G. Breyer, Clarence Thomas, Samuel A. Alito Jr., and Sonia Sotomayor.

Justice Antonin Scalia dissented on the grounds that the law as written by Congress was "hopelessly vague." He has consistently objected to criminal laws that do not clearly spell out the crime.

Justices Elena Kagan and Ruth Bader Ginsburg dissented separately. They said Sykes was convicted of "simple vehicular flight," not a high-speed or dangerous flight that truly qualifies as potentially violent.

Read the justices' opinions in the violent-felony case via http://go.philly.com/sykes