PASADENA -- A man suspected of decapitating a Cal State L.A. professor was a co- worker with an extensive and violent criminal history.

Mark Stephen Guerrero, 38, of West Covina, who apparently killed himself Monday morning, served time in state prison before earning a master's degree in education and working at the school, officials said.

Contrary to police reports, Guerrero and Glenda Vittimberga, 37, were never in a romantic relationship, her friends said.

Guerrero is the primary suspect in Vittimberga's brutal death in her Pasadena home.

She was a much-loved professor of education at Cal State L.A. and her killing shocked the campus community, school officials said.

"She was intelligent, kind, had a great sense of humor and was well-liked on campus by students, faculty and in the field,' said associate professor Margaret Garcia, who shared an office with Vittimberga.

Vittimberga, who had a Ph.D in clinical psychology, specialized in teaching students to work with developmentally disabled children, Garcia said.

Garcia said she was with Vittimberga and her family on Saturday, and Vittimberga spent Sunday with friends.

Police suspect Guerrero stabbed Vittimberga multiple times and beheaded her in the early hours of Monday morning.

Guerrero, described by those who knew him as handsome and charming, then stripped naked at 3:51 a.m. and stepped in front of a tractor-trailer truck on Interstate 15, killing himself, authorities said. Police do not think he sexually assaulted Vittimberga.

Guerrero is the primary suspect in Vittimberga's death, but until tests on blood found in his car are completed, detectives cannot rule anything out, police said.

The blood, believed to be Vittimberga's, would link Guerrero to the crime, police said.

Colleagues of Vittimberga and Cal State L.A. school officials say they knew nothing about Guerrero's lengthy rap sheet. Such information is not kept in the enrollment records, school officials said.

Guerrero was convicted of nine criminal counts in October 1996, including attempted murder, assault and battery on a police officer, leaving the scene of an accident and driving under the influence after a May 12, 1995, incident in Glendale, said Jane Robison, a spokeswoman for the Los Angeles County District Attorney's office.

He served one year in two state prison camps for the 1995 offenses, said State Department of Corrections spokeswoman Doris Mahlun. He was on parole until October 2000, Mahlun said.

Guerrero who sometimes used the alias Mark Knight also was arrested for other offenses, police said. In 1987, Glendale police arrested him for battering a police officer, said Sgt. Tom Delgado of the Pasadena Police Department.

In 1986, West Covina police charged Guerrero with misdemeanor assault with a deadly weapon after he beat up the boyfriend of his former girlfriend, said West Covina police Commander Mark Tedesco.

In that incident, Tedesco said Guerrero confronted the woman at home, tried to rip off a ring he had previously given her as a gift and assaulted her boyfriend when he intervened, police records show.

Garcia said Vittimberga knew Guerrero had a felony conviction because he was not eligible for a teaching credential although he had completed his master's degree in education at Cal State L.A. in 2000.

Guerrero and Vittimberga knew each other because she supervised his work as a test librarian in the school's diagnostic resource center, Garcia said.

He had been accepted to pursue a master's degree in psychology and took classes as recently as winter 2002, school officials said.

Guerrero wanted to date Vittimberga, but she limited it to friendship, Garcia said, and was cautious about being too friendly with Guerrero in case he misinterpreted her actions.

Vittimberga was supportive of the apparent turnaround in Guerrero's life, Garcia said. He did not drink or smoke and had recently converted to Catholicism Vittimberga also was a Catholic after trying several other religions, Garcia said.

"She wanted to be supportive,' Garcia said. "She wanted him to continue his education.'

People acquainted with Guerrero said they would never have suspected him of violent crimes. Guerrero's relatives, reached at his last known residence, in the 3800 block of Morganfield Avenue in West Covina, declined comment.

Hema Davis at Post Box Plus in Walnut, Guerrero's last known mailing address, said he was very friendly to her when he came to pick up his mail. She described him as a "very charming, very personable' customer who always paid his bills on time.

A statement by the university described Vittimberga as a "rising star' who was promoted to full professor two years ago.

"The university has lost a young and vibrant scholar a woman who dedicated her life to the education of others,' the statement said.