Irvine Police Department Press Release

Farrah Emami - Public Information Officer
1 Civic Center Plaza
Irvine, CA 92606



Subject :

Suspect Identified in Irvine Homicide
Contact :Bill Whalen, Commander - Crossroads Area    949-724-7025

Irvine, CA – The Irvine Police Department (IPD) has identified the suspect involved in a homicide on an Irvine bike path. The suspect is identified as Dennis Thomas Monson, Jr., age 37, transient.   

In the early morning hours of Jan. 19, 2016, Monson was with 51-year-old victim Sidney Siemensma, with whom he was acquainted, on a bike path adjacent to the Interstate 405 near Irvine Center Drive behind the Los Olivos residential community.  An altercation occurred, and Monson repeatedly stabbed the victim. The suspect then fled the scene.

At approximately 7:00 a.m., IPD received a report of a bleeding man from a jogger, followed by similar reports from passing motorists. IPD responded and located Siemensma, who was determined to be deceased.

Detectives began investigating the case and quickly identified Monson as a possible suspect. That night at approximately 7:20 p.m., an IPD officer patrolling the area located and contacted the suspect on the same bike path where the homicide had earlier occurred.

It was subsequently determined that Monson was in possession of child pornography on his cell phone. The suspect was charged for the child pornography by the Orange County District Attorney (OCDA) and has been in custody on those charges since Jan. 19, 2016.

While Monson remained in custody, IPD continued investigating the homicide, including submitting evidence for forensic testing. Based on the evidence in the case, IPD has identified Monson as the suspect in the murder of Siemensma and the case has been submitted for criminal charges to the OCDA. 

It is believed that Monson is the sole suspect in this case and there are no outstanding suspects. 



One Civic Center Plaza, Irvine, CA 92606
(949) 724-7000

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