The suspected gunman in the fatal shooting of a Los Angeles County sheriff's deputy pleaded not guilty by reason of insanity to murder with special circumstances.

Kevin Salazar, 29, was arrested on Monday in what authorities described as the "ambush" shooting of Deputy Ryan Clinkunbroomer.

During his arraignment on Wednesday, Salazar's attorney entered a plea of not guilty and a dual plea of not guilty by reason of insanity on his behalf. He is being held without bail and is scheduled to next appear in court on Nov. 7 for a preliminary hearing.

Clinkunbroomer, 30, was on duty, in uniform and in his patrol car, when he was shot while stopped at a red light outside the Palmdale Sheriff's Station Saturday night.

Clinkunbroomer was "ambushed by a coward," Los Angeles County Sheriff Robert Luna said Monday.

Amid the search for the suspect, investigators said they were seeking a Toyota Corolla as a vehicle of interest in the case. Sharing details on the Corolla led investigators to the suspect and the car, the sheriff said.

Salazar was arrested Monday morning after barricading himself inside a house for several hours, Luna said.

Multiple law enforcement sources told ABC News Monday night that Salazar confessed to investigators.

The motive remains unclear.

Investigators are trying to determine whether the suspect had an officially documented history of psychiatric problems and whether that would have or should have triggered red flags that might have stopped him from buying a gun.

Salazar's mother told ABC News that she had reported concerns over her son's mental health to law enforcement, but it remains unclear whether any such reports would have risen to a level that should have blocked her son from being able to legally purchase a gun. The Los Angeles County Sheriff's Department has not commented on whether Salazar's mother contacted them about her son's mental health.

No other suspects are believed to be involved, the sheriff said.

Clinkunbroomer, an eight-year veteran of the sheriff's office, was "a dedicated, hard-working deputy sheriff," Clinkunbroomer's family said in a statement read on Monday by the sheriff.

"Ryan was recently engaged to the love of his life," his family said, and he made "the ultimate sacrifice."

ABC News' Emily Shapiro and Josh Margolin contributed to this report.