More people are getting killed in Philadelphia, and fewer of those killings are getting solved.

Those are the takeaways from crime data for 2015: After two years of historically low homicide numbers, Philadelphia - like many other U.S. cities - saw its murder rate climb. There were 277 murders here last year, a 12 percent increase from 2014.

The rate at which homicides are solved by investigators has dropped - police made arrests in about 52 percent of killings in 2015. As recently as 2013, the figure was 71 percent.

Nonfatal shootings in the city also increased, by 18 percent.

Commissioner Charles H. Ramsey - whose last day in office was New Year's Eve - said no single factor explained the increased homicides and decreased clearance rate this past year. A spike in drug-related killings, plus witnesses who are reluctant to come forward, may have played a part, he said.

But the retiring commissioner pointed out a positive: 2015 was the third year in a row that murders in the city have stayed below 300. It's the first such three-year stretch since 1969, he said.

"I'm always sad about the fact that it's gone up a bit," he said. "But I don't think it's time to panic."

Killings spiked in several sections of the city, Ramsey said - "we had some districts that got off to a really bad start." Those include the 19th District in West Philadelphia and the 24th District, which covers Port Richmond and parts of Frankford and Kensington, long a hub for drug sales.

Drug-related killings increased by 55 percent in 2015, Ramsey said.

"That's really something to be concerned about," he said. He said those killings ranged from drug dealers fighting over turf to people who were shot trying to purchase drugs. It wasn't yet clear what had caused that spike, he said.

In addition, killings that police determined had been motivated by arguments increased by about 17 percent, he said.

Homicides increased nationally last year, in some cases much more markedly than in Philadelphia. Smaller cities such as Milwaukee, St. Louis, and Baltimore - where the per-capita murder rate for 2015 was the highest in that city's history - saw dramatic spikes, while Chicago and New York saw lesser upticks.

"We'll keep doing what we can to drive it down," Ramsey said. "I'm sure that when Deputy Ross takes over, that'll be his focus, to push the numbers down."

Deputy Commissioner Richard Ross, Ramsey's longtime second in command, is to be sworn in this week as commissioner.

Ramsey said he can't point to any specific reason that fewer homicides were solved in 2015. He said it remains difficult to persuade murder witnesses to come forward, and "it's hard to believe that people don't know who's doing this stuff out here."

"If we don't get the information" to arrest murderers, he said, "they're going to continue to be out there doing what they do, and commit another crime or be killed themselves."

New department policies put in place in 2014 limited how long detectives can hold suspects and required investigators to tell witnesses, victims, and suspects that they are allowed to leave interviews at any time. But Ramsey said he didn't think the new rules had contributed to the drop in the clearance rate.

Still, he said, "we have to make sure that we're protecting the rights of individuals, and that's more important than a clearance rate. I'd rather have a low clearance rate than know that we have abused anyone's rights."

Overall, what the FBI calls Part One crimes - which encompass violent crimes and serious property crimes - decreased by about 5 percent last year, according to statistics released by the department through Dec. 27.

And some crime categories saw significant decreases in 2015 - residential burglaries decreased by 16 percent, and commercial burglaries were down 22 percent. Robberies also decreased slightly.

"You can't deploy [officers] just for violence," said Deputy Commissioner Kevin Bethel, who, like Ramsey, is retiring from the department. "You have thefts, burglaries, robberies - your strategy has to be comprehensive. There are a lot of good things going on, and overall, crime has continued to go down."