Surveillance images show Rivers leaving the scene, according to Kent Police Chief Ken Thomas.
"We believe it was Mr. Rivers that pulled the trigger," Thomas told KIRO 7. "We certainly know, if Mr. Rivers' sentence wasn't commuted, he wouldn't be out of custody and he would not have committed that crime."
David A. Cabrera was found dead in motel room No. 18, according to a police report. He had been "shot in the face" in front of his girlfriend during what police call a suspected drug deal.
Theneious Fisteral Swafford, 47, has been charged as an accomplice in the case. Investigators believe Swafford, also a convicted felon, drove Rivers to and from the crime scene.
Rivers' lengthy police record includes multiple convictions for robbery, assault and "a propensity for violence that creates a substantial likelihood of danger to the community," according to charging documents filed this week in King County Superior Court.
Rivers' criminal history was used as an example by policymakers drawing up the state's three-strikes law in the 1990s, according to Paul Guppy of the Washington Policy Center.
"He became one of the first cases in our state that was eligible for 'three strikes, you're out,'" Guppy told KIRO 7 on Tuesday. "He was sentenced to life, so the system worked, and for every day he stayed in jail, the public was safe."
"In this case, he was released and he committed another violent crime," Guppy said. "It's a stunning failure of a system that was put into place by the voters of our state to protect the public from exactly this kind of person."
KIRO 7 reached out to then-Gov. Gregoire for comment. This is the statement her former chief of staff, Marty Loesch, released:
"King County Prosecutor Dan Satterberg, the Clemency and Pardons Board and the sentencing judge, Kathleen Learned, recommended the commutation of the sentence of Mr. Stonney Marcus Rivers to me in 2013. Their recommendations carried great weight with me. Mr. Rivers had been sentenced to life without possibility of parole soon after the passage of Washington's Persistent Offender Accountability Act, the so-called "Three Strikes Law" for the crime of Robbery 2 and by the time of his release had served twenty years in prison. Robbery 2 typically results in a sentence of two to three years. Based on these recommendations and the facts available at the time, to include his performance while in prison, I conditionally commuted Mr. Rivers sentence to his time served subject to compliance with twenty-one different conditions. The allegations against him suggest that he violated these conditions and should be returned to prison for the rest of his life. My heart goes out to the victim, family and friends."