The incident began as a break up between a 28-year-old Novato man and his 22-year-old girlfriend, who lived in a house just outside Petaluma with her mom and brother and grandfather.
Nine days ago the woman broke up with Thomas Halloran. He had a criminal record for threatening people and sent a text to the girl on Monday, threatening to kill her family.
Halloran frightened the girl and her family to the point that they called a friend -- Mathiesen. Mathiesen came to the house to try and calm the situation and 20 minutes after he arrived, Halloran showed up carrying a gun.
"Mathiesen went outside to see and calm down the suspect as he approached the house; at that time the suspect shot Mathiesen twice in the upper torso, killing him at the scene," Assistant Sonoma County Sheriff Lorenzo Duenas said.
Investigators say Halloran then grabbed his former girlfriend's mother in an attempt to take her hostage. The girl's brother grabbed a gun of his own.
"He then went around to the outside of the garage area where the mother was being forced into a vehicle and shot the suspect twice once in the lower extremity, the leg," Duenas said.
Another shot to the head killed Halloran.
Halloran and Mathiesen were dead when sheriff's deputies arrived.
Mathiesen was a nine year veteran of the Marin sheriff's department. Before that he had worked as a contractor. He is survived by a wife and two grown sons.
Investigators say Halloran has a long rap sheet in both Somona and Marin counties. He did eight months behind bars for threatening other people and had restraining orders against him from another former girlfriend and an ex-wife.
Investigators are treating Halloran's death as a justifiable homicide and they are continuing to investigate his history. They say Halloran may have had gang ties, and they are not releasing the name of the ex-girlfriend's brother for fear of possible retaliation.