LANCASTER, Calif. (KABC) -- The suspect in the ambush killing of a Palmdale sheriff's deputy entered a dual plea of not guilty and not guilty by reason of insanity in a Lancaster courtroom Wednesday.
Kevin Cantaneo Salazar, 29, was charged with one count of murder.
District Attorney George Gascón said the murder charge includes three special-circumstance allegations: the crime was committed against a peace officer; lying in wait; and discharge of firearm from a motor vehicle.
If convicted, Salazar would likely face a sentence of life in prison without possibility of parole, Gascón said. The district attorney repeated his previous opposition against seeking the death penalty in general.
"If I thought that seeking the death penalty was gonna bring Ryan back to us, I will seek it without any reservation," Gascón said. "But it won't."
"If I thought the death penalty was gonna stop people from committing brutal murders, I will seek it. But we know that it won't. The reality is that the death penalty doesn't serve as a deterrent and the death penalty does not bring people back."
In court, Salazar was seen with his arms restrained to his hips.
He has reportedly confessed to killing Deputy Ryan Clinkunbroomer, multiple law enforcement sources told ABC News.
The deputy's fiancee, Brittany Lindsey, spoke through tears at Wednesday's news conference.
"Ryan was the best guy I've ever met," Lindsey said.
"He was so thoughtful, caring. Everyone who met him or knew him, loved him. I'm so happy I was able to love him. It was not long enough. I couldn't wait to start our lives together. We were just engaged. Planning to get married and start a family."
"Ryan, I miss you and I love you so much. I don't know how to live without you. And I didn't ever want to imagine it.
A GoFundMe has been established to help Clinkunbroomer's family and fiancee.
WATCH: Fallen deputy honored in procession
Salazar was arrested at his Palmdale home Monday morning after a lengthy standoff with sheriff's deputies. They found a significant number of firearms inside the home.
Investigators believe Salazar drove up behind Deputy Clinkunbroomer's patrol vehicle and ambushed him with gunfire Saturday night. A passerby found the deputy unconscious in his car and alerted authorities. He was pronounced dead at the hospital.
Salazar's family said he has mental-health issues which include schizophrenia and hearing voices in his head.
His family says they understand Salazar will have to face the consequences. But they said his alleged actions may have been the product of mental illness.
"We just want you guys to know my brother has schizophrenia, he's bipolar," said the suspect's sister, Jessica Salazar. "He was diagnosed with schizophrenia. A lot of people that have family members out there are going to understand."
"He would feel persecuted, voices talking to him. He tried committing suicide once or twice."
When the plea of not guilty by reason of insanity was announced in the courtroom, several deputies present were heard gasping and muttering.
Gascón says his office has evidence to dispute the insanity claim. He showed awareness when he purchased the gun weeks before the killing and he held a job, the district attorney noted in an interview with ABC7's Marc Brown for Eyewitness Newsmakers.
"We know a lot of things about him that indicate to us that he was aware of the import of his actions," Gascón said. "And we have charged the case accordingly. But we'll let the evidence come out."
Salazar is being held without bail. His preliminary hearing is scheduled for November.
City News Service contributed to this report.