A Kentucky man who was the youngest of six defendants to plead guilty to the killings of a Knox County family in 1997 now wants a federal judge to throw out his sentence.

Lawyers for Jason B. Bryant, 34, argue U.S. Supreme Court decisions from 2014 and 2016 make clear his current sentence of life without the chance for parole is unconstitutional.

Lawyers Daniel Horwitz of Nashville and James P. Danly of Washington filed a petition for a writ of habeas corpus May 18 in U.S. District Court in Nashville.

Bryant is serving his life sentence plus a consecutive 25-year term at the Turney Center Industrial Complex in Only, Tenn., southwest of Nashville.

Bryant and five fellow Kentuckians pleaded guilty in a take-it-or-leave-it deal in Greene County in 1998 to the killings of Vidar and Delfina Lillelid and their daughter Tabitha, age 6. The young defendants met the family at a rest stop off Interstate 81, threatened them with a gun and forced them to go to a quiet nearby road, where the victims were shot to death.

The Lillelids' son Peter, age 2, was shot but survived and now lives in Sweden.

Bryant was 14 at the time. The others blamed him for the killings; Bryant said he killed no one but was told by the others that he'd take the fall for the crimes because of his youth.

The six, all of whom had lawyers, pleaded guilty to the killings. All received sentences of life without a chance for parole, which they are now serving at various prisons in the state.

Two recent cases - Miller v. Alabama and Montgomery v. Louisiana - decided by the U.S. Supreme Court sharply limit the circumstances under which a defendant under age 18 can be sentenced to life without a chance for parole. The rulings can be applied retroactively to past sentences.

Juveniles already are spared the threat of the death penalty in the United States.

The Montgomery ruling states that a term of life without parole can't apply to young convicts "but for the rarest of juvenile offenders, those whose crimes reflect permanent incorrigibility," the lawyers argue.

Further, they say, there's a presumption under the Miller ruling that juveniles aren't to receive sentences of life without a chance at parole.

Bryant's lawyers argue the new cases apply precisely to their client. They say Bryant was "intellectually stunted" and had the "social IQ of an 11-year-old" at the time of the killings.

"As a consequence, Mr. Bryant has the distinction of being the youngest person in the state of Tennessee -- either before or since -- to be condemned to die in prison," the petition states.

Bryant's lawyers in April released an online statement that expressed sorry for what had happened to the Lillelids but criticized some of his co-defendants for debating the case in public. Bryant killed no one, the lawyers argue.

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They cannot say more while the federal petition is active, an email from Horwitz to 10News states.

"Out of respect for the judicial process and all parties involved -- as well as the Rules of Professional Conduct that apply to cases of this nature -- my co-counsel and I have also resolved not to comment further until his case has concluded," the emailed statement reads.

Co-defendant Karen Howell also was a juvenile - 17 - at the time of the Lillelid killings. She was present but did not shoot anyone, she insists. She pleaded guilty, however, to murder.

The Greene County judge who handled the Kentuckians' guilty pleas in 1998 is no longer on the bench.

Last month, Greene County Circuit Court Judge Alex Pearson heard arguments from Howell's lawyer that the Supreme Court decisions apply to his client. Pearson, however, disagreed, finding that the sentence as applied by the original judge in the plea deal was appropriate.

Howell's attorneys say they'll appeal.

Bryant's lawyers say the federal judge has the option of granting Bryant a parole hearing, of resentencing him to a term that gives him a "meaningful" chance at release or to a new sentencing hearing "that gives full and fair consideration both to the factors set forth in Miller and to the presumed constitutionality of a sentence of life without the possibility of parole established by Montgomery."