For friends and family of Susan Lowe, Thursday’s guilty plea by a violent sex offender finally offered the answer that has eluded them for 34 years.

Michael Allan Halgren, 58, who has been committed as a violent sexual predator for the past 12 years, pleaded guilty to second-degree murder, admitting he strangled Lowe in her Bellevue apartment in 1980.

He faces a maximum sentence of life in prison when he is sentenced July 18.

Lowe’s roommate, Diane Dietz, discovered her body on April 24, 1980. She said the slaying was “a horrible lightning bolt out of the blue” and she has long wondered who would kill her friend.

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“I finally have an idea of what happened. Up until this day it was inexplicable,” Dietz said by phone from her office in Eugene, Ore., on Thursday. “I had to learn to put one foot in front of another, raise a family and have a career.”

Although a reward was offered for information on Lowe’s killer, the case went cold until 2012, when scientists from the Washington State Patrol Crime Laboratory matched DNA found in Lowe’s bedroom with Halgren, according to charging documents.

Halgren’s DNA was obtained by investigators while he was confined at the Special Commitment Center on McNeil Island, where he was civilly committed in 2002.

Halgren was initially charged with first-degree murder in January 2013. Senior Deputy Prosecutor Kristin Richardson said Thursday that the DNA evidence had degraded over time, prompting prosecutors to reduce the charge in exchange for a guilty plea.

“These issues are typical in cold cases,” Richardson explained. “We have no doubt it was him.”

Halgren, who lived a short distance from Lowe and Dietz, is believed to have been a stranger to the 19-year-old victim, prosecutors said. Investigators said they may have frequented the same Eastside bowling alley.

Lowe was spending a quiet night alone at home with plans to watch a Sonics game when she was killed, authorities said. Dietz, who was out studying with friends, said their apartment was quiet when she returned and she assumed Lowe had gone to bed.

When Lowe wasn’t up by 8 the next morning, Dietz went into her room to make sure she wouldn’t be late to work. Lowe had been strangled with a pair of pantyhose, authorities said.

A neighbor told investigators a loud knock was heard on Lowe’s front door and a banging noise “like something hitting the wall.” Neighbors also reported hearing a muffled scream while they were watching TV’s “Charlie’s Angels” between 9:30 and 10 p.m.

Halgren had been confined to the Special Commitment Center after he was convicted of first-degree rape for a 1989 sexual assault on a woman in Bellevue; and of unlawful imprisonment after he abducted a prostitute in Seattle in 1995, according to court records.

Doctors at the center learned over the years that Halgren had peeped through women’s windows when he was a teen, exposed himself to nearly 40 women when he was in his early 20s and raped more than 20 women, mainly prostitutes, after being discharged from the military in 1975, according to the charges in the Lowe case. He also told doctors he raped a relative after graduating from high school and raped three more women in 1974, 1979 and 1980, according to charging paperwork.

When Halgren was first housed in the Special Commitment Center, DNA was not routinely taken from patients, according to Tacoma police Detective Lindsey Wade, who attended Thursday’s plea hearing because her work led to Halgren’s DNA being taken.

Now DNA samples are always taken from convicted felons and sex offenders living at the center if they haven’t already been taken by the state Department of Corrections, she said.

Lowe’s family didn’t attend Thursday’s plea hearing in King County Superior Court, but Richardson said they’re happy the case is resolved.

Since the slaying occurred when inmates in Washington state were subject to parole, it will be up to the Indeterminate Sentence Review Board to determine when Halgren is released on the murder conviction. He faces a sentence of between 13½ years and life in prison.

After his release from prison on the murder conviction, Halgren will return to the Special Commitment Center to continue his sexually violent-offender treatment until he is deemed unlikely to reoffend, Richardson said.

The Lowe case was the first homicide sent to the King County Prosecutor’s Office Cold Case Unit when it was created in 2006, Richardson said.

Prosecutor Dan Satterberg said Thursday there “is a special satisfaction when we can solve a murder more than three decades old using modern forensic science.”

Dietz, who is a reporter at the Eugene Register-Guard, wrote in an email that Lowe was “a beautiful young woman who was mischievous and funny.

“She’d tear up at GE commercials — and then laugh at the silliness. She watched the SuperSonics and Lost in Space. She was fascinated by the then-growing bulge on Mount Saint Helen’s flank. She spent a whole night with her roommate getting the melody and harmony to ‘Today While the Blossoms Still Cling to the Vine’ just right.”

Information from Seattle Times archives is included in this story.

Jennifer Sullivan: 206-464-8294 or On Twitter @SeattleSullivan.