Transient accused of killing man on Irvine bike trail found mentally incompetent to stand trial

A 41-year-old transient has been found mentally unfit to stand trial for the stabbing death of a man on a popular Irvine bike trail.

An Orange County Superior Court Judge on Friday ordered that Dennis Thomas Monson Jr. be committed to a state hospital for mental health treatment, court records show.

The judge’s finding that Monson is not currently mentally competent essentially places his murder case, along with a related case alleging child-porn possession, on hold until it is determined he is able to understand the charges he is facing and assist with his own defense.

In the morning hours of Jan. 19, 2016, a jogger called 911 to report a bleeding man on the San Diego Creek Trail, near the 405 Freeway and the Los Olivos Apartment Community.

Authorities identified the apparent stabbing victim as 51-year-old Sidney Siemensma, a stay-at-home dad and avid bicyclist.

Monson, who was identified by police as an acquaintance of Siemensma’s, was quickly arrested, initially on charges of child-porn possession after officers searched his cell phone. Monson was later charged with murder in connection to Siemensma’s death.

Authorities did not specify a motive for the suspected altercation that led to Siemensma’s death.

According to testimony from investigating officers during a February 2018 preliminary hearing, Siemensma often rode the Irvine bike trails during the evening and night hours. Monson had acknowledged talking to Siemensma the night of his death, the officers testified.

Investigators testified that they found a jacket with blood stains at a tent they believed Monson used in a wooded area near the bike trail, according to a court transcript. DNA tests confirmed that both Monson and Siemensma’s blood was on the jacket, a forensic scientist from the Orange County Crime Lab testified at the preliminary hearing.

Irvine detectives also spoke to the occupants of a van that stalled out on the nearby 405 Freeway the night of Siemensma’s death, who reported hearing the sounds of an argument, a fight and screams, according to the court transcript.

At the close of the preliminary hearing, a judge ruled that there was enough evidence for Monson to stand trial for the murder charge. The criminal proceedings were put on hold in September, after Monson’s attorney raised questions about his mental competency.