WASHINGTON - The Supreme Court has limited the Constitution's protection against double jeopardy in cases involving multiple charges and a deadlocked jury. In a 6-3 decision, the court ruled Thursday that prosecutors may try again to convict a defendant of murder even after jurors in a first trial vote to acquit the defendant of murder -- but is split on whether to convict on a lesser charge of manslaughter.

Chief Justice John Roberts, speaking for the court, said a defendant is protected against retrials only when the jury has rendered a "final decision" on the charges against him or her.

The case arose from the 2007 death of a 1-year-old Arkansas boy, Matthew McFadden Jr., from a head injury while he was at home with his mother's boyfriend, Alex Blueford. The prosecution said Blueford had slammed Matthew into a mattress; Blueford said he had accidentally knocked the boy to the floor.

Blueford was charged under four theories: capital murder, first-degree murder, manslaughter and negligent homicide. The jurors were to consider the most serious charge first and move to the next only if they agreed unanimously that Blueford was not guilty. In this way, they were to work their way to the appropriate conviction, or to an acquittal.

The jury unanimously agreed that Blueford was not guilty of capital or first-degree murder but was divided, 9-3, in favor of guilt on the manslaughter charge. The court declared a mistrial.

The high court said the Constitution does not protect a defendant from being tried again for murder. "The jury in this case did not convict Blueford of any offense, but it did not acquit him of any either," Roberts said. "As a consequence, the double jeopardy clause does not stand in the way of a second trial."

Justices Antonin Scalia, Anthony Kennedy, Clarence Thomas, Stephen Breyer and Samuel Alito agreed.

In dissent, Justice Sonia Sotomayor faulted the court for weakening the historic protection against double jeopardy. This rule "unequivocally prohibits a second trial following an acquittal," and the trial judge should have ruled he had been unanimously acquitted on the murder charges, she said. "This case demonstrates that the threat to individual freedom from re-prosecutions that favor states and unfairly rescue them from weak cases has not waned with time. Only this court's vigilance has," she wrote. Justices Ruth Bader Ginsburg and Elena Kagan agreed with her.

The New York Times contributed to this report.