Nationally, the percentage of homicides that go unsolved has skyrocketed, even as homicide rates have fallen to the lowest they've been since the 1960s. Five decades ago, about 90 percent of homicides were solved, compared with fewer than 63 percent today.
The trend is especially disturbing, experts say, considering that detectives have far more technological tools, such as DNA testing and ballistics analysis, to solve crimes.
Philadelphia's crime-fighters say they buck the national trend: The city's homicide clearance rate stands at about 71 percent - lower than the recent high of 75.5 percent in 2009 but higher than the 60.2 percent it dipped to in 2011, according to police data.
Homicide clearance rates, by the FBI's definition, include arrests made on old, cold cases. Looking at arrests made just for murders that happened in 2013, Philly's clearance rate is 40 percent.
Lt. John Stanford, a police spokesman, referred questions about Philly's homicide-clearance rate to Homicide Unit Capt. James Clark. Clark did not return several calls for comment.
Nationally, the blame for declining homicide-clearance rates can be pinned on several trends, experts say.
Witness intimidation and the "no snitch" culture mean fewer witnesses cooperate with police, while municipal budget cuts and a focus on homeland security translate to less money for police departments, said Bill Hagmaier, executive director of the International Homicide Investigators Association and a former FBI profiler.
Killers also have grown more sophisticated - and therefore, tougher to catch - thanks to "how-to" true-crime books, TV shows and the Internet, Hagmaier added.
America's gun culture has also contributed, said Jerry Ratcliffe, a criminal-justice professor and director of Temple University's Center for Security and Crime Science.
"As a nation, we have higher homicide rates and lower homicide-clearance rates than any westernized country, because we have almost nonexistent gun regulation," Ratcliffe said. "We allow people to kill each other at some distance. It's pretty difficult to have a drive-by stabbing, but it's really easy to have a drive-by shooting."
Standardizing training for homicide investigators and refocusing on killings instead of terrorism could help improve clearance rates, Hagmaier said.
"Ma and Pa Kettle are more worried about the next Ted Bundy than they are about the next Osama bin Laden," he said.
Tipsters with information in Don Ly's killing or any other Philadelphia homicide should call detectives at 215-686-3334, or contact the police tip line by calling 215-686-8477 or texting PPD TIP (773847).