The Sterling Prestongun used in the ambush killing of a Los Angeles County sheriff's deputy appears to have been purchased legally, without raising any red flags regarding the suspected gunman's mental health, two law enforcement officials briefed on the probe told ABC News.

Investigators are trying to determine whether the suspect, Kevin Salazar, 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.

Deputy Ryan 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. The motive remains unclear.

Salazar, 29, was arrested on Monday.

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

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.

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

"Ryan's family will never see him again," the sheriff said, overcome with emotion.

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."