Marin Sheriff's Deputy Killed in Petaluma

An off-duty Marin County sheriff's deputy was shot and killed early this morning by the ex-boyfriend of a female friend during a domestic violence incident near Petaluma, according to Sonoma County Assistant Sheriff Lorenzo Duenas said.

The incident was reported at about 12:15 a.m. at a home in the 800 block of Liberty Road.

Investigators believe the deputy, Jim Mathiesen, 49, was killed by 28-year-old Thomas Edward Halloran before Halloran was fatally shot by a third man at the home, Duenas said.

Mathiesen, a Petaluma resident who had been a deputy for nine years, was friends with a female resident of the home on Liberty Road who had been dating Halloran since March, Duenas said.

Mathiesen, who worked in the Marin County Jail division of the Sheriff's Office, is survived by his wife and two children, according to Sheriff's Lt. Barry Heying. He began his stint at the Marin Sheriff's Office at the age of 40 after a career as a contractor.

Duenas said the relationship between Halloran and the woman was rocky, and that Halloran had sent death threats by phone and over the Internet to the woman after she broke off the relationship nine days ago.

Mathiesen had helped the woman, a family friend, in the past with her troubles with Halloran, and he went to the Liberty Road house after Halloran texted her a death threat around 11:30 p.m. Monday, Duenas said.

Halloran then showed up at the house. The fatal confrontation occurred when Mathiesen went outside the house to try to calm Halloran down, Duenas said.

Halloran shot Mathiesen twice in the upper torso, and Halloran's ex-girlfriend fled, Duenas said.

Halloran then allegedly took another woman who was living in the house hostage and forced her into a car, Duenas said.

Another male who was living in the house then shot Halloran in the leg and upper torso in defense of the female hostage, Duenas said. Mathiesen and Halloran were both pronounced dead at the scene.

“This is tragic news, Jim [Mathiesen] died trying to help others,” Sheriff Bob Doyle said in a statement. "He was known for his kindness and his helpful nature."

Family, friends and employees throughout the sheriff’s office are grieving today with many sharing stories of how Jim touched their lives with his generosity.

“This is a time of great sadness for us all and right now our focus needs to be on taking care of the family Jim left behind, and that includes Jim’s Sheriff’s Office family as well,” Undersheriff Timothy Little told sheriff’s employees at an early morning briefing.

Halloran was on parole for making threats in the past against other victims, and the Sonoma County Sheriff's Office is investigating whether he had gang ties or affiliations, Duenas said.

"He had numerous arrests in Marin and Sonoma counties," Duenas said.

Both weapons used in the shootings were recovered, Duenas said. 

Halloran's ex-girlfriend is safe, and is being interviewed by sheriff's investigators, Duenas said.

--Bay City News Service contributed to this report.

Petaluma Patch is pursuing independent information about the local family involved in this tragedy. You can