Two families from the Crow Indian Reservation in Montana can proceed with a lawsuit against an F.B.I. agent that accuses him of failing to properly investigate crimes against American Indians on and around the reservation, the United States Supreme Court has ruled.

The court declined last month to reverse a 2010 federal court ruling that said the F.B.I. agent, Matthew Oravec, did not have qualified immunity from legal action, a protection usually given to government employees when acting in an official capacity — and a status sought by the Justice Department, which had appealed the ruling by the Court of Appeals for the Ninth Circuit.

“The decision puts federal and state law enforcement agents on notice that they may be held personally liable if they discriminate against Indians in investigating crimes against them,” said Patricia S. Bangert, a Denver lawyer who is representing one of the families.

The Supreme Court’s decision was dated Jan. 14, but lawyers were only recently made aware of it.

Mr. Oravec, who remains an F.B.I. employee, investigated the deaths of two men, Robert Springfield and Steven Bearcrane, who died in unrelated episodes on the reservation in 2004 and 2005, respectively.