Transient held in stabbing of Irvine man on bike trail


Investigators walk a bike path near the 405 Freeway in Irvine where a body was found Jan. 19. A suspect has been identified in the case.

(Don Leach / Daily Pilot)

Police have identified a man in custody on unrelated charges as a suspect in the killing of an Irvine man on a bike path last month. 

Dennis Thomas Monson Jr., a transient who frequents the bike path near the Los Olivos neighborhood, was arrested the evening that Sidney Siemensma, 51, of Irvine, was stabbed to death, the Irvine Police Department said Wednesday in a news release.

Siemensma’s body was discovered about 7 a.m. Jan. 19 on a bike path that runs adjacent to the southbound 405 Freeway south of the 133 Toll Road, according to police.

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Authorities received calls from passersby who thought a bicyclist on the path had been injured, Irvine police spokeswoman Farrah Emami said at the time. But when emergency crews arrived, they found Siemensma’s body. 

Dennis Thomas Monson Jr.

Irvine Police have identified Dennis Thomas Monson Jr., a 37-year-old transient, as a suspect in a man’s stabbing death.

(Irvine Police Department)

Police said Wednesday that a man with whom Siemensma was acquainted was “quickly identified” as a possible suspect. According to the news release, police believe Siemensma was stabbed repeatedly during an altercation between the two.

The same day as the stabbing, Monson, 37, was contacted by police on the bike path. Officers discovered what they believed to be child pornography on Monson’s cellphone, officials said. He has been in custody on child pornography charges since Jan. 19, authorities said.


Detectives continued their investigation and identified Monson as the suspect in the stabbing, the department said.

Brittany Woolsey writes for the Daily Pilot.

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