For more than 20 years, Robert McCallister dutifully did his police work despite being in near-constant pain.

The Pennsylvania cop's torment was a constant reminder of the bullet that became lodged in his spine during a 1981 gun battle with a bank robbery suspect.

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McCallister, known as “Bo,” endured numerous surgeries and medical complications throughout his life as a result of the shooting, and, on Jan. 20, he died at the age of 69. A forensic pathologist said the 1981 gunshot caused his death, prompting Susquehanna Township Police Chief Robert Martin to open a murder investigation.

“He was the greatest police officer I ever worked with,” Martin said of McCallister, who retired from the police department in 2003. “Even after being shot in 1981, he persevered and returned to duty with a bullet lodged in his spine. He is a testament to what a hero is in our country.”

For Martin, McCallister was an important role model, both in his personal life and as a professional.

McCallister was the first sergeant Martin worked with, and as a rookie officer, Martin absorbed how masterfully his mentor communicated with the public. McCallister, who had the distinction of being the youngest police officer promoted to sergeant, taught the younger members of the department about being humane -- but also stressed the importance of being prepared for the more trying and dangerous parts of the job.

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“He taught us about mental preparation, he was always [focused on] keeping officers’ safety paramount during traffic stops, and crimes in progress,” Martin said.

Those he mentored were nearly oblivious to the pain McCallister was grappling with, and the sergeant preferred it that way.

“With us young guys he didn’t talk about it much,” Martin said. “He had surgeries -- throughout his career, he had to take off for surgeries, but you never would have known” about the health complications, otherwise.

“He was still doing his job and not complaining,” Martin said.

In a 1994 interview with the Patriot News, McCallister spoke about how the devastating shooting had given him a new perspective on what his priorities should be.

“The job was my life; my family came second [before the shooting],” McCallister said. "I’m thankful to be alive. Now the most important thing in my life is my family. You can take everything away, but as long as I have my children and my wife and my health, nothing else matters."

He said the shooting changed the simple everyday routine of leaving for work.

“They know what I think,” he said of his wife and children. “And I know what they’re thinking: ‘I hope he’s coming back.’”

The job was my life; my family came second. I’m thankful to be alive. Now the most important thing in my life is my family. You can take everything away, but as long as I have my children and my wife and my health, nothing else matters. — Sergeant Robert McCallister, in a 1994 interview with the Patriot News

The person who shot McCallister fled after firing the life-altering shot and was never charged with what, at the time, would have been aggravated assault.

McCallister didn’t let that consume him, Martin said.

“He was able to compartmentalize it, but there’s no question it never left his mind,” he said.

Among the myriad honors McCallister received in his career, he earned the Medal of Valor and Purple Heart due to his heroism during the 1981 bank robbery.

When he left the department 16 years ago, he looked forward to the increased time he would get to spend with his wife and children.

“I remember him explaining to me like it was yesterday,” Martin said. “He said, ‘My career was one chapter in the book of my life, and it’s time to move to the next chapter.’”

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That next chapter included quality time with his loved ones.

“He spent a lot of time with his family, he traveled and did some odd jobs,” Martin said.

Now that the shooting case is a homicide, there is no statute of limitations -- meaning if investigators figure out who shot McCallister, the assailant can be brought to justice for his death.

“We’re digging out reports that are in dusty boxes, there’s a lot of work to do,” Martin said. "[McCallister] deserves everything we can give to find who did this. It’s for him and for his family, who endured the pain, and for our profession – we’re sending the message that we don’t forget, we’ll work to the very end to find who shot a police officer.”