RIVERSIDE, Calif. — An outpouring of grief continued on Tuesday for a slain California Highway Patrol officer who was killed in a shootout with a gunman in Riverside that left two fellow officers wounded.

CHP Officer Andre Moye Jr. was doing paperwork to impound a pickup truck just off the 215 Freeway about 5:30 p.m. Monday when the driver reached into the vehicle, suddenly grabbed a rifle and opened fire, authorities said.

Other CHP officers responded and joined what Riverside's police chief described as "a long and horrific gun battle," followed by police officers and deputies from nearby agencies. The gunman was killed in the confrontation, whose motive remains unknown.

"Our hearts ache over the tragic loss of Officer Andre Moye - who will be remembered for his commitment to service and the community of Riverside County," Gov. Gavin Newsom said on Twitter.

Family members said Moye was 33 and had been with the Highway Patrol for about four years.

CHP Commissioner Warren Stanley said he was "devastated by the tragedy that unfolded earlier in Riverside."

On social media, hundreds of people paid tribute to Moye, whose body was escorted from a Moreno Valley hospital to the coroner's office by a phalanx of law enforcement officers.

"R.I.P. CHP Officer Andre Moye. Prayers and deepest sympathies to the family, friends and colleagues," Pablo L. Torres wrote on Facebook. "Prayers for the two injured Officers as well."

The CAHP Credit Union has also established a memorial fund in honor of Officer Andre Moye, Jr.

In a press release, the credit union said that one-hundred percent of all donations received will go directly to Officer Moye's family. The CAHP Credit Union is covering all processing fees and administrative responsibilities.

In an interview, the gunman's father identified him as 49-year-old Aaron Luther, adding that the son recently seemed depressed, was having knee pain and marital problems.