A 2010 study by Scripps Howard News Service found that of more than half a million killings between 1980 and 2008, almost 185,000 went unsolved.
Experts such as Bill Hagmaier, a former FBI agent who heads the International Homicide Investigators' Association in Florida, cite a few reasons.
Funding cuts have eroded training and manpower in many detective units, meaning fewer resources for cold cases, he said. At the same time, television shows, movies, and the Internet offer the public a broader understanding of police techniques, possibly helping killers cover their tracks.
Even officers are not immune to the "CSI effect," which leads juries to expect DNA evidence and fingerprints, Hagmaier said.
"As veteran investigators are retiring, you're seeing a new crop come in," Hagmaier said. "Some of these officers expect that all these forensic tools are going to make their job a lot easier when it comes to solving crimes, when the truth is that's not always the case. DNA is great, but it's not everything, and it's not a substitute for the best old-school techniques."
Some experts also cite the changing nature of violence in big cities over the last few decades, saying gang- and drug-related killings can be more difficult to solve than crimes of passion, especially amid a rising "no-snitch" culture.
A study last year by the U.S. Department of Justice found police departments that work at building relationships with the community are better equipped to solve homicides. Not surprisingly, the study also found departments that use best practices for investigating homicides, such as allocating overtime pay to allow officers to work around the clock, have the most success with closing cases.
In Philadelphia, the homicide clearance rate dipped below 56 percent in 2006. When Commissioner Charles H. Ramsey took over the department in 2008, he appointed a new homicide chief, reorganized the internal structure of the units, and instituted a weekly review and brainstorming session on open cases.
Over the next several years, the department's clearance rate fluctuated between 60 percent and, at one point, close to 80 percent.
In Chester, Bowen said more help from the community where her sister was killed would help the officers working on the case.
"It takes a lot to go to the police and say, 'I know who did this,' " Bowen said. "I'm not sure everybody is strong enough to do it."
Hayward, an Annapolis native, was 28 when she came to Pennsylvania with her fiancé, who worked in the area. Days later, on Feb. 12, 2004, Bowen says she believes, an argument led Hayward to check into a Days Inn near Philadelphia International Airport shortly before 2 a.m.
She was later seen getting into the elevator with a man. Delaware County District Attorney Jack Whelan said she called the front desk soon afterward and said she had a problem, but no one went up.
A hotel employee found her body in the bathtub the next morning. Police found two cigarette butts floating in an ice bucket, and DNA from a black male. Hayward's fiancé was cleared as a suspect.
"This is not the type of case that usually remains unsolved," said Whelan, whose office recently reviewed the case file. "It's somewhat unusual to go this long and not be able to match the DNA."
Often police cannot predict which cases will go cold. In the days after two armed men, one wearing a Halloween monster mask, burst into Philip Kistler's West Chester office and shot the doctor in a botched robbery in 1994, authorities were hopeful the killers would be caught.
But more than 20 years have passed, and though police have reported new leads in the case, it remains open.
Other cases have confounded authorities from the start, such as that of 51-year-old lawyer Eric Birnbaum, shot execution-style outside his Bucks County office in 2009. Birnbaum was chatting with a secretary in the parking lot around 9 a.m. when a man wearing sunglasses and a knit hat walked up, fired a bullet into the back of his head, and fled.
Police had many potential leads: former clients, people Birnbaum had sued, former romantic partners, a neighbor he argued with.
Nothing panned out.
Rumors emerged of gambling problems, drug addiction, and possible ties to the Russian mob, said Terry Goldberg, Birnbaum's longtime friend and former law partner.
"None of the theories make any more sense than the others," said Goldberg, who had been friends with Birnbaum since they were teenagers. "At this point, I always come back to, could it have been a flat-out mistake? Part of this is wanting to clear his name, so maybe I just want to believe that."
The 2005 death of Michael Ewer in Lansdale drew less media attention than Birnbaum's. But it has remained a top priority for investigators, said Kevin Steele, Montgomery County first assistant district attorney.
Ewer, 49, who had mental health problems and liked sleeping outdoors, was beaten to death Oct. 20, 2005, in a baseball dugout in Memorial Park. He was found in his sleeping bag.
Police found footprints and blood spatter nearby, but little other evidence. Authorities have interviewed more than 100 people, chasing leads as far away as New Orleans.
"You look at this case, what happened here, and you wonder, 'What else is this person capable of doing?' " Steele said. "Or, what else have they done?"
Technology has helped
Technology has led to arrests in decades-old homicides around the region. Last year, a DNA match allowed police to arrest the killer of Anjeanette Maldonado, a 17-year-old strangled in North Philadelphia on her way to school in 1996.
In 2001, a national fingerprint database linked a man to the murder of John McManus, who had been stabbed in his Cheltenham home 14 years earlier.
Without such breaks, the passage of time makes it harder to solve an open case. Memories fade. Eyewitnesses disappear.
Some cash rewards are posted for decades, said Santo Montecalvo, vice president of the Citizens Crime Commission of Delaware Valley, which collects anonymous tips.
"It depends on when people break off alliances," he said. "You've got boyfriends and girlfriends who have a spat, and they call us."
But an arrest does not always bring the catharsis families long for.
"There's a relief, but it wears off," Montecalvo said. "If my son was murdered, tomorrow or the next day when I wake up, I'm not going to be thinking about how that guy who killed him is in jail. I'm thinking about how I'll never see my son again."
Goldberg has considered the possibility that learning why Birnbaum died might traumatize him anew, particularly if the reason is something he does not expect.
"There's a lot of endings that wouldn't be good for me," Goldberg said. "But I still want an ending."
BY THE NUMBERS
Percentage of homicide cases nationwide in the 1960s in which an arrest was made
Percentage of murder cases nationwide today in which an arrest is made
Unsolved murders nationwide between 1980 and 2008
SOURCE: FBI and Scripps Howard News Service