A Utah police department is grieving for a fellow officer slain while working overtime to pay for his cancer treatments amid questions about why his shooter was on the streets in the first place, despite a long rap sheet.

Officer Douglas Scott Barney, of Unified Police of Salt Lake County, was killed Sunday after coming across Cory Lee Henderson, who later died in a shootout with other officers.

Henderson, 31, had walked away from a state-run parolee drug treatment center last month. It was his latest cycle through federal prisons and halfway houses as he faced multiple gun and drug-related charges over the past decade.

Gang investigators had tracked him to a Salt Lake City suburb Friday night, but missed capturing him, said Lt. Lex Bell of the.

"They're taking it a little bit hard," Bell said.

Henderson had run a red light and crashed a car Sunday, then walked away. Barney, 44, who was responding after the crash, was found shot in the head, his gun still in the holster.

The father of three teenagers was working an overtime shift he'd taken to help pay off debt from his bladder cancer treatments. He had been a police officer 18 years.

Henderson also shot one of Barney's partners through both legs before he was killed. Officer Jon Richey, 51, was released from a hospital on Tuesday after his wounds were treated.

Officers didn't know Henderson was a fugitive until hours after the shooting, Bell said. Police are often frustrated to see violent offenders out of jail weeks after arrest, he said.

"We'll risk our life to find somebody. A month or two later, they go back on the street," he said.

Henderson had been paroled from prison in April, after serving 14 months on a gun charge. He got out early after finishing a drug treatment program.

He was picked up on federal firearm and drug possession charges months later, and U.S. Magistrate Judge Evelyn Furse ordered him to a state-run treatment center for parolees as he awaited trial last month, according to court records.

That's not unusual in such cases, said Randy Richards, president of the Utah Association of Criminal Defense Lawyers, speaking generally.

Within days, Henderson checked out to look for work and never came back.

He was with a woman during the Sunday car crash. She has since talked to police, but wasn't arrested or publicly identified since she could be a witness. She wasn't with him at the time of the shooting, police said.

Police are planning a candlelight vigil to honor Barney and Richey for Wednesday, and Barney's funeral is set for Monday.

Some 10,000 people are expected at the funeral for one of the first officers in the country to die on duty this year, and the first ever for the Unified Police Department since it formed in 2010 to serve communities in the Salt Lake City area.

Bell had worked closely with the slain officer and said the two shared Twizzlers candy and their memories of their first patrol jobs last week. Even people he put in handcuffs appreciated Barney's sense of humor, his colleague said.

"He's got them laughing before they ever go to jail," Bell said.

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Ho reported from Las Vegas.