The streets of Philadelphia see more than their fair share of murders. But even when a killer's identity is known, it's no guarantee detectives will be able to make an arrest, due to the police department's struggles with a lack of witness cooperation.

There were 277 murders in Philadelphia in 2015, a slight uptick over last year, and homicide detectives had a 52 percent rate of cases in 2015 being closed with an arrest, a slight decrease.

Outgoing police commissioner Charles Ramsey said one big obstacle to solving cases is non-cooperation by witnesses. It doesn't just not help police, he said — it could actually be causing more violence.

"Our problem is that, even when we're sitting on a 52 percent clearance rate, in probably 80 to 85 percent of the cases, we have a good idea of who committed the crime. We just don't have enough to bring them to trial," said Ramsey at his final press conference last week. "You can't bring charges forward if you don't have people willing to testify."

When Ramsey took over the department, he and Mayor Michael Nutter instituted the city policy of offering a $20,000 reward for information leading to the arrests of suspects in any murder in the city —but that hasn't made a significant change to the problem, he said.

"That's a lot of money we put up for that, and it's still not been able to raise our numbers to the point where we really have people stepping forward in large numbers," Ramsey said.

RELATED: Family mourns death of high school grad Gregory Burnside III

"Those people [murder suspects] stay on the streets, and one of two things happens. They commit another crime, or they become a victim," he said.

That's exactly what Denene Coleman, the mother of 21-year-old Paris Coleman, who was murdered one year ago, said happened in her son's case.

"The child that was accused of Paris' murder was murdered a couple of months down the line," Denene told Metro. "It's just a never-ending cycle."

Coleman's case is still officially listed as unsolved, a police spokeswoman said. He was shot to death blocks from his Brewerytown home, near Thompson and Hollywood streets, in broad daylight on a Sunday afternoon, on Jan. 11, 2014.

Denene said she always knew who the killer was — a former friend of Paris' and associate in the drug trade who had accidentally shot her son before.

She was questioned by police after the alleged killer died, in part because her car was stolen and used in the murder, she said — but she has no idea what exactly happened, other than a belief that he was killed by people who knew her son.

She said detectives asked her repeatedly, "Is it over?" But Denene said she knew nothing about the murder, and didn't want the case to end this way.

"Him being in the ground is not going to satisfy anybody — not my family, not his kids," she said. "There's no closure. Closure would be him being locked up behind bars for the rest of his life."

Denene has since moved to Georgia. 

"It seemed like everywhere I went just reminded me of Paris," she said.

RELATED: Family of Paris Coleman hopes for justice after daylight murder

Another case listed unsolved is that of Gregory Burnside III, 18, murdered during an altercation in broad daylight a block away from his Southwest Philly home.

His father, Gregory Burnside Sr., said still sees the men he believes killed his son every day on the corner selling drugs.

"Nothing happened with them," he said. "Nothing at all. They're still selling drugs around the corner and they're still coming back and forth as they please."

Burnside III, a recent high school grad, was gunned down on Glenmore Ave near 61st Street in broad daylight on Jan. 7 after an argument that Burnside Sr. believes was sparked by perceived disrespect. Numerous neighbors witnessed the shooting, he said, but none of them have gone to police.

"The whole block was witnesses, but nobody will come forward," he said.

As far as getting by with his grief, he relies on his faith, family, and support from the families of other crime victims. He is planning a celebration of his son's life on Friday night at the corner where he was killed.

"I'm not that unique. I know a lot of people going through the exact same thing as me," Burnside said.

"It's living in Philadelphia. I've been here all my life and it don't seem that strange to me. You just never think it will happen to you."

Additional reporting by Jenny DeHuff