ATLANTA (CNN) -- Autopsy results released Friday revealed that Ryan Brunn, the Georgia man who this week admitted murdering a 7-year-old girl, committed suicide by hanging himself in a prison cell, a state agency said.

The Georgia Bureau of Investigation, in a news release, said the autopsy indicated that Brunn died of "ligature hanging and the manner of death (was) suicide."

The 20-year-old hung himself with his sweatshirt, investigators determined. A medical examiner didn't find any "other significant trauma on Brunn's body."

"The ongoing investigation by agents ... has not uncovered any evidence to indicate Brunn's death was anything other than a suicide," the state investigative bureau said.

Georgia Department of Corrections spokeswoman Kristen Stancil had said that Brunn was found unresponsive at 4:15 p.m. Thursday in his cell at Georgia Diagnostic and Classification Prison in Jackson, and was pronounced dead at a hospital at 5:37 p.m.

In a Cherokee County courthouse on Tuesday, Brunn pleaded guilty to a charge of murder in the death of young Jorelys Rivera in Canton, Georgia, about 40 miles north of Atlanta. He was sentenced to life in prison without the chance of parole, avoiding a potential death sentence by lethal injection.

A maintenance worker at the same apartment complex where the girl lived, Brunn said in court that he devised a plan to lure Jorelys -- whom he said he'd never talked to before -- after he found one of her skates.

On December 2, 2011, Brunn took a picture of the skate and approached the girl with it, asking if the skate was hers. She said yes, and Brunn promised to take her to it.

Instead, he took her to an empty apartment and instructed Jorelys to pull down her pants. He then put tape over her face and cut her throat with a razor. At this point, she was still alive, he told the court.

Brunn said that he then took Jorelys to the bathroom and beat her to death with the skate. He said he did not have sex with the girl.

The girl's mangled body was found in a trash compactor three days after she went missing. Two days later, Brunn was arrested. He was eventually indicted on 13 counts including murder, aggravated assault, aggravated child molestation, enticing a child for indecent purposes and more.

"I'd like to apologize for everything I've done, and I do deserve everything that you're about to give me," he said Tuesday in court.

Jorelys' father , Ricardo Galarza, told CNN en Español on Thursday that news of Brunn's death partly made him "feel good but, on the other hand, it doesn't because my daughter will never be with me again."

"What I wanted was for them to put him in the electric chair and burn him," Galarza said. "I would have taken him there myself and prepared it, so he would burn."