Tony Harris, 50, of Philadelphia, was murdered in October. The case remains unsolved. 
Tony Harris, 50, of Philadelphia, was murdered in October. The case remains unsolved.  - 

Tony Harris was one of 280 people murdered last year in Philadelphia.

“You know, the streets talk, said Harris’ brother-in-law, Lloyd Williams. "We kinda have an idea of who the people are. But it's not enough evidence for the police to be able to make an arrest. Something to make it stick.”

Harris’ case is among nearly half of the murders in Philadelphia last year that went unsolved. Authorities there say that’s largely because people won't cooperate with police. City officials have been offering cash rewards in exchange for information. But the results so far are mixed. 

Police call the percentage of killers they catch the "homicide clearance rate," and cities with a low rate have more killers walking the streets.

“Clearing cases saves lives,” said Thomas Hargrove, a retired investigative journalist who runs the Murder Accountability Project

He said the clearance rate has been dropping nationwide. One big reason?  More mistrust of police.

West Virginia University professor Rachael Woldoff said high-profile cases nationwide of unarmed black men shot by police hasn’t helped.

“It’s not just individual African-Americans. It’s about the neighborhood context and the way police behave in African-American communities,” she said.

Jennifer Selber, Philadelphia’s lead homicide prosecutor, said she would like to see more cooperation from those who have information about murders.

“It's extremely frustrating,” Selber said. “Especially because when we actually arrest people, our conviction rate is over 90 percent.”

The city offers $20,000 to anyone with information that leads to a homicide arrest.

“Well, there's some people that $20,000 is motivation to come forward and share information,” said Philadelphia’s District Attorney Seth Williams.

For some people in the community, though, cooperation may look more like “snitching.” 

“There's some people that $20,000 is not enough. Because they're so worried about retaliation, or being shot, murdered, having their car keyed,” Williams said.

Since 2013, Philadelphia's paid out more than $600,000, according to records obtained from the city.

Meanwhile, the DA's office has other creative techniques. “They've even had people that want to look like they're being arrested, so that they're looking like they're being taken down against their will, and they're not being cooperative,” Selber said.

Yes, some people won't talk unless police stage an arrest.

Lloyd Williams said he plans to hang posters around his community in Southwest Philly to spread the word about cash rewards.

But the underlying issue of trust to help solve a murder, prosecutors say, might take decades to repair.