MADISON, Wis. (AP) -- A college dropout pleaded guilty Monday to killing a stranger in the victim's home, a crime that unnerved residents and remains something of a mystery.
The dropout, Adam Peterson, acknowledged that he walked into Joel Marino's home near the University of Wisconsin-Madison campus and stabbed him in the abdomen and chest on Jan. 28. Marino, a musician and employee of a medical device company, was found dying in a nearby alley.
Despite the plea, the reason Peterson went to Marino's home that day remains unclear. The two did not have any previous relationship.
Prosecutors have hinted that Peterson was attempting a burglary, but the families of both men said Monday the homicide remained unexplained.
Peterson, 20, who dropped out of UW-Madison last year, did not make any statement in court.
He has been diagnosed as psychotic and schizophrenic but was able to tell right from wrong, said his lawyer, Dennis Burke. "Mr. Peterson was sane and rational at the time he committed this crime," he said.
Dane County Circuit Court Judge James L. Martin said Peterson will face mandatory life in prison when he is sentenced in the next few months. The judge will decide whether Peterson should qualify for parole but said Peterson must spend at least 20 years behind bars.
Prosecutors said they would ask that Peterson be imprisoned for 40 years before he could seek supervised release.
Marino's death, unsolved for months, combined with two other unsolved and high-profile murders in the city prompted many residents to take safety precautions.
Peterson was arrested in June, after he showed up on a Police Department list of potentially mentally ill residents, and police found that he matched the description of a man seen fleeing Marino's home on the day of the stabbing.
Authorities said Peterson confessed, and police said his DNA matched a sample taken from the 4-inch paring knife used to kill Marino.
Peterson, of Stillwater, Minn., has been on suicide watch in jail since September, when he tried to hang himself.
The victim's father, Louis Marino, said he would urge the judge to impose the maximum punishment.
"The sadness will never go away," Marino told reporters after the hearing. "He destroyed the life of my family."
Peterson's father, Melvin Peterson, called the guilty plea "a step toward closure" for both families.
"Hopefully I'll find out one day what was going on in his mind," Melvin Peterson said.