Riverside police today are mourning the death of an officer who was shot to death Sunday night while attempting to stop a hit-and run-driver, authorities said.

Officer Ryan Bonaminio, a four-year veteran of the force, died at a hospital, Riverside police Sgt. Jaybee Brennan said. His age was not released.

Police sealed off some areas in the vicinity of Fairmount Park alongside the Moreno Valley (60) Freeway and Market Street to search for the man believed to have shot and killed the officer, Lt. Leon Phillips said. The search was continuing after daybreak, Brennan said.

Bonaminio had pulled the suspect's truck over at about 9:45 p.m. on Sunday in front of Fairmount Park, at Market Street and Fairmount Boulevard, Brennan said.

"There was a foot pursuit into the park,'' Brennan said. Backup officers following the foot pursuit heard shots and found the officer down, she said.

The downed officer was rushed to a local hospital, where he died. The suspect's description is only a male in dark clothing, Brennan said. Extra officers have been called in to aid in the search, Phillips said.

Anyone with information on the case was urged to to call police Detective Ron Sanfilippo at (951) 353-7105, or Detective Gary Rowe, (951) 353-7130.