BILLINGS, Mont. — A U.S. judge ordered a convicted killer from Montana's Crow Indian Reservation released from prison on Thursday after part of his conviction was overturned for not meeting the definition of a violent crime.

Assistant U.S. Attorney Lori Harper Suek wept as she told the court, including the victim's three daughters, that she had no legal basis to ask to keep Quinton Birdinground Jr. behind bars.

Birdinground served 15 years of his original 24-year sentence for assaulting his estranged girlfriend and killing his uncle, Emerson Pickett, after a night of drinking in 2003.

U.S. District Judge Susan Watters expressed unease over Birdinground's release but said she was bound under a 2015 Supreme Court ruling and encouraged prosecutors to appeal.

"'How in the world could second-degree murder not be a crime of violence?' I get that," Watters said to Pickett's family during Thursday's hearing. "I have to follow the law."

Birdinground was found guilty of assault and murder, but the issue hinges on his conviction of using a firearm during a violent crime.

Watters threw out the firearm conviction last month, citing the 2015 ruling and saying the U.S. law behind the weapons charge was so vague as to be unconstitutional.

She had written that neither second-degree murder nor assault is itself considered a "crime of violence" under federal law. That's because the assault and murder may have been committed recklessly rather than intentionally, as is required to be considered a violent crime.

The legal explanations behind Birdinground's release provided scant solace for Pickett's family.

His 72-year-old aunt, Myra Lefthand, said she had always assumed Birdinground would remain in prison until after she died. Pickett's daughter, Jena, said the release brought back all the pain she felt when she was 13 and her father was killed.

"We're not healed, and we're never going to heal," Jena Pickett said.

Under his new sentence, Birdinground will be supervised by a probation officer for the next five years. He expressed no remorse over the killing but said he would "try to stay as far away as I can" from the reservation.

He was ordered to have no contact with the victim's family and to remain in Billings for now. Pickett's daughters live across the street from the defendant's mother on the Crow reservation and were worried that he would want to return home.

Suek, the prosecutor, pledged to keep pursuing the case and try to send Birdinground back to prison.

"We do believe Mr. Birdinground will be back in the courtroom, and we will be able to revive the original sentence," she said.

Prosecutors in Montana will be consulting with the U.S. Solicitor General's Office to decide on an appeal, said Leif Johnson, first assistant U.S. Attorney for Montana.

The full ramifications of the 2015 Supreme Court ruling are still being determined, and it's already come up in other cases.

The justices overturned an enhanced sentence given to Samuel Johnson, a white supremacist whose crimes included being a felon in possession of a firearm. Writing for the majority, Justice Antonin Scalia said Johnson's right to due process had been violated because the 1984 federal law on which his sentence was based included sections so vague that the law could be misused.

Numerous appeals are pending to determine how broadly the ruling applies to other federal laws that use the same or similar language to define a violent felony, the federal prosecutor in Montana said.