By Sara Patterson, Douglass Dowty and John O'Brien

Syracuse, NY - David Renz has been promised a lifetime in prison after admitting today to the murder of Liverpool school librarian Lori Bresnahan and the sexual abuse of a 10-year-old girl.

But that plea could be withdrawn if federal prosecutors decide to seek the death penalty against Renz for the fatal carjacking, said County Court Judge Thomas J. Miller.

Renz, 29, admitted sexually assaulting the girl after using a knife to cut off her clothing. He also admitted stabbing Bresnahan multiple times and attempting to strangle her.

Renz said in a prepared statement he did not know what caused him to commit such savagery.

"I'm truly sorry for my actions and I'm prepared to accept the consequences for them," Renz said.

David Renz

Renz pleaded guilty this morning to first-degree murder and predatory sexual assault against a child. He was promised a sentence of life in prison without parole on the murder conviction and 25 years to life as a sex offender for the assault charge.

Both sentences will run concurrently, or at the same time, Miller said.

"Under this agreement, you would never be released from prison," the judge told Renz.

One motivation for Renz to plead guilty was the hope that it would get federal prosecutors to back off on possibly pursuing the death penalty, said Federal Public Defender Lisa Peebles, who was in court for the plea.

In May, the U.S. Attorney's Office notified Renz's lawyers that they were considering indicting him on a federal carjacking charge and whether to pursue the death penalty against him.

"It was a big reason why he pled," Peebles said. "There were other reasons too - he wanted closure. But we felt this was the best way to avoid the death penalty."

Bresnahan's family has said they wanted today's guilty plea to be the final disposition of the murder and sexual abuse charges, Peebles said.

Senior Assistant District Attorney Matt Doran speaks during the sentencing of David Renz in Onondaga County Court Wednesday.

Executive Assistant U.S. Attorney John Duncan wouldn't comment on the impact Renz's plea might have on the death penalty decision.

"In any federal case where the death penalty is a possible outcome, there is an established protocol that must be followed," he said.

Renz also agreed to lead investigators to the whereabouts of the pellet gun used in the crime. He will also be interviewed by the FBI Behavioral Sciences Unit.

After court, his lawyer, Kenneth Moynihan, said he didn't think taking the case to trial would have helped.

"The evidence was overwhelming," Moynihan said. "It would have been very difficult based on the facts."

Renz ambushed Bresnahan and the child on March 14 as they were leaving a gymnastics practice at the Great Northern Mall. Before the attack, Renz slipped off an ankle monitor he had been fitted with in January on a federal child pornography charge, according to police.

The 10-year-old girl was tied up and sexually assaulted, and Bresnahan was stabbed to death in a struggle to save the girl's life, according to Onondaga County District Attorney William Fitzpatrick.

Photos from the crime scene search, investigation

The 10-year-old managed to escape from the car and was found running down Verplank Road by two passersby who happened upon the scene, called authorities and comforted Bresnahan in her final moments, according to Fitzpatrick.

The girl did not witness the murder, said Fitzpatrick.

Renz was indicted on charges of first-and second-degree murder, first-and second-degree kidnapping, predatory sexual assault, predatory sexual assault against a child, first-degree sexual abuse, second-degree menacing, criminal possession of a weapon in the fourth-degree and possessing a sexual performance by a child.