Authorities said the incident began after the 27-year-old police officer attempted to pull over an 18-wheeler truck at the Market Street exit off the 60 freeway about 9:45 p.m. Sunday. The truck was believed to be involved in a hit-and-run accident.
Bonaminio pursued the vehicle, which did not yield until Fairmount Park, at which point the suspect got out of the vehicle and fled on foot. Bonaminio got out of his squad car and chased the suspect. The suspect turned and fired at Bonaminio.
"The driver yielded in front of the park, and ran from the vehicle. The officer followed in foot pursuit. A short time later, shots were heard, the officer was struck," said Riverside Police Sgt. Jaybee Brennan.
When backup officers arrived, they found Bonaminio with at least one gunshot wound. He was rushed to a local hospital where he died a short time later from his injuries.
According to the Riverside Police Department, the suspect was described as a black male mid-30s to mid-40s, 6 feet 1 inches to 6 feet 2 inches tall, with slender build and possible facial hair. The suspect was last seen wearing dark clothing and a light-colored baseball cap.
The suspect is wanted and believed to be armed with a handgun. He is considered armed and dangerous.
The department released a photo of the suspect taken during the incident.
Bonaminio was a four-year veteran of the Riverside Police Department and had also served two tours in Iraq with the U.S. Army.
Capitol flags in Sacramento were flown at half-staff in honor of Bonaminio on Monday.
"Officer Bonaminio went above and beyond the call of duty his entire life, first serving in uniform to defend our nation's freedoms and here at home to protect the people of Riverside," said Gov. Arnold Schwarzenegger in a statement. "His courage and dedication in the line of duty will be remembered forever."
Riverside Police Chief Sergio Diaz released a statement: "Officer Ryan Bonaminio's tragic death is a reminder to all of us in the law enforcement family that the supreme sacrifice of our service is also a cost borne by our loved ones, our families, and our community."
The suspect remained at large Monday.
A volunteer team with metal detectors arrived on scene around 7 a.m. Monday looking for the murder weapon or other evidence. Shell casings and keys were found.
"They said they had found all the casings. All we're looking for now is the weapon, and the weapon is still evading us," said Bob Bowes, a volunteer searcher.
Anyone with information related to the case is urged to call Riverside Police at (951) 787-7911.
Bonaminio is survived by his parents and a brother and sister.
A memorial fund has been set up for Bonaminio:
Riverside Police Officers Association
In care of: Ryan Bonaminio
1965 Chicago Ave, #B
Riverside, CA 92507
The Associated Press contributed to this story.