(CNN) -- A former Los Angeles police officer who police say has violently threatened his former colleagues shot at least two officers early Thursday, killing one, sparking a huge manhunt in Southern California, according to authorities.
Christopher Jordan Dorner, 33, is also wanted in a double slaying Sunday in Irvine, California.
The California Highway Patrol issued an alert Thursday morning urging officers in several Southern California counties to be on the lookout for Dorner after the overnight shootings.
Two officers in Riverside were shot, one fatally, police there confirmed to CNN. CNN affiliate KTLA reported another officer was shot in Corona, California.
Irvine police identified Dorner on Wednesday as the suspect in the February 3 deaths of Keith Lawrence and Monica Quan in Irvine.
Dorner also has made violent threats against other Los Angeles police officers, the agency said Wednesday.
"I will bring unconventional and asymmetrical warfare to those in LAPD uniform whether on or off duty," Dorner allegedly wrote in a lengthy letter promising retribution against the department, where he worked from 2005 until 2008. An LAPD source provided the document to CNN.
The letter writer claimed he was terminated for reporting excessive force by a fellow officer, and said his attacks were retribution for his termination, as well as a culture of racism and violence he says continues within the department.
He called the attacks against police "a necessary evil that I do not enjoy but must partake and complete for substantial change to occur within the LAPD and reclaim my name."
Los Angeles police warned people who encounter Dorner not to approach him, saying he is likely "armed and extremely dangerous."
"Our department is implementing all measures possible to ensure the safety of our LAPD personnel, their families and the Los Angeles community, and will continue to do so until Dorner is apprehended and all threats have been abated," police said in a statement.
CNN's Mallory Simon and Deanna Hackney contributed to this report.