Marin Sheriff's Deputy Killed in Double Homicide Near Petaluma

An off-duty Marin County sheriff's deputy was shot and killed early this morning by the ex-boyfriend of a female acquaintance during a domestic violence incident near Petaluma, Assistant Sonoma County 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, who had been a deputy for nine years, was a family friend with the residents of the home on Liberty Road. The 22-year old young woman of the family had been dating Halloran since March, Duenas said.

Duenas said the relationship 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.

About 20 minutes after Mathiesen arrived, according to reports, Halloran 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.

According to Lt. Barry Heying of the Marin Sheriff's Office:

Jim [Mathiesen] lived in Petaluma, he is survived by his wife and two children.

Jim began with us at age 40 after a career as a contractor. He spent nine years working in the jail as a deputy sheriff.

Halloran then allegedly took the young woman's mother. who was also living in the house, as a prospective  hostage and tried to force her into a car, Duenas said.

Another male who was living in the house, identified by Sonoma county sheriffs as the brother of the young woman, armed himself and 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.

A press release from the Marin County Sheriff's Office reads in part as follows:

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

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” said Undersheriff Timothy Little addressing 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. Halloran was also said to be on active state parole for making terrorist threats in the past.

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.

A later press conference at the Sonoma County Sheriff's Department in Santa Rosa clarified some of the relationships and actions of the event (see attached video).

From Bay City News Service and other sources as noted.

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

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