Dennis Thomas Monson Jr. I.D.'d as Alleged Killer of Sidney Siemensma

Dennis Thomas Monson Jr. was acquainted with the murder victim, police say.
Dennis Thomas Monson Jr. was acquainted with the murder victim, police say.
Irvine Police Department

A 37-year-old transient who has been in custody on child pornography possession charges since Jan. 19 is also a suspect in the same-day murder of a married, stay-at-home father of two in Irvine, according to police.

Dennis Thomas Monson Jr. was acquainted with 51-year-old victim Sidney Siemensma, according to a statement from Irvine Police Commander Bill Whalen.

In the early morning hours of Jan. 19, Siemensma, an avid cyclist, was on a bike path adjacent to the 405 freeway near Irvine Center Drive and behind the Los Olivos residential community, where he got into an altercation with a man, Whalen said.

The fight escalated to Siemensma repeatedly being stabbed before his attacker fled, according to Whalen, who adds IPD received reports from a jogger and passing motorists around 7 a.m. that a man was lying on the bike path bleeding.

When officers arrived at the scene, Siemensma was already dead, Whalen said. The victim leaves behind a wife, a 14-year-old son and a 10-year-old daughter.

Sidney Siemensma, R.I.P.
Sidney Siemensma, R.I.P.
Siemensa family

Detectives quickly identified Monson as a possible suspect, and an officer patrolling the bike path area confronted the transient around 7:20 p.m., Whalen said.

Because Monson had child pornography on his cell phone, he was taken into custody and later charged with possession by the Orange County District Attorney's office, Whalen said.

Meanwhile, detectives continued working their homicide and based on forensic evidence they have collected since Monson was taken into custody, they have concluded he is their murder suspect, Whalen said.

The case has been submitted to the district attorney for criminal charges, according to Whalen, who added there are no other suspects in the homicide.


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