By Greg Argos

PHILADELPHIA (CBS) — Who murdered Officer Frederick Cione? It’s a question the Philadelphia Police Department has never been able to answer.

The rookie officer was shot and killed 47 years ago this week.

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CBS 3’s Greg Argos took a close look into the killing, which began with a routine police stop on a cold January 30, 1970 morning. Cione was just 25 at the time.

“He’s in his patrol car and what we believe is he gets out to stop three African-American males,” said Philadelphia Police Capt. James Clark.

Cione approached the three men near 17th and West Oxford shortly after 1 a.m.

It would turn out to be the rookie cop’s last assignment as he was ambushed and shot multiple times.

“He goes down,” said Clark. “He stands over top of him and shoots him again.”

Now 47 years later, and 26 boxes of homicide files, interviews and evidence gathered, Cione’s killer has never been arrested. His murder is the only unsolved police killing in Philadelphia Police Department history.

“I have a cold case unit which is a lieutenant, a sergeant and eight detectives and all they do is deal with cold cases, and obviously this is one of them,” said Clark.

Clark runs the department’s homicide unit and knowing a fellow officer’s murderer is still free has bothered him for years.

“We try obviously to solve every homicide, but when one of ours get killed, it’s even more important for us to try to get it solved and it’s a nagging feeling that we have one police officer killed in the history of the police department and it hasn’t been solved,” said Clark.

Cracking this case is very personal for him.

“His brother worked with me for many, many years in civil affairs, and even more so than that, my late father as a police officer actually worked on this case 40-something years ago,” explained Clark.

However, leads have vanished in the case.

“With this case, we really didn’t have any witnesses,” said Clark.

Tips stopped coming in and the department never tracked the killer.

“Detectives did their best to run on them, but they always hit an impasse or a dead spot,” said Clark.

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The big question though is why was Cione killed.

“I couldn’t help but placing this murder, but it sounds like an assassination to me in historical context,” said La Salle sociology professor Charles Gallagher.

Gallagher has one theory and it has to do with race and police relations around the time of the killing.

“So it’s 1970. Six years prior, we have one of the worst race riots we ever had in Philadelphia,” said Gallagher.

Those riots in North Philly were just blocks away from where the killing later happened – 17th and West Oxford. This was also a time when the city was tense about police and community relations.

“At this time in Philadelphia history there was sadly lots of DOJ reports of police brutality,” said Gallagher.

The sociology professor thinks the rookie cop may have been a target simply because he was wearing blue.

“The times of the moment were that police officers were targets. If you were a white police officer in a black neighborhood, you could be singled out,” said Gallagher. “This is one theory. Just the attitudes and the way things were at the time and I believe that that had a lot to do with why this unfortunately happened.”

“I do believe there are people that are still out there today who knew who did this,” said Clark.

Clark is still hoping for the one call with that one piece of information.

“We’ll run on whatever information they give us,” said Clark, adding that it is “a very special case that I would love to be able to bring in.”

CBS 3’s Greg Argos also spoke with Cione’s only brother, Nicolas Cione, a retired Philadelphia police sergeant.

He did not want to appear on camera, but has another theory about what happened to his brother.

He believes his brother’s killer was alone and wanted on a warrant, which is why he shot and killed the officer when he approached.

Cione said it was a cold night, which is why there were likely no witnesses, and believes that’s why it’s been so hard to solve this case.

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Reward for information leading to the killer’s arrest is set at $40,000.