Every year in America, 6,000 killers get away with murder.

The percentage of homicides that go unsolved in the U.S. has risen alarmingly, even as the homicide rate has fallen to levels last seen in the 1960s.

Despite dramatic improvements in DNA analysis and other breakthroughs in forensic science, police fail to make an arrest in more than one-third of all homicides. National clearance rates for murder and manslaughter have fallen from about 90 percent in the 1960s to below 65 percent in recent years.

The majority of homicides now go unsolved at dozens of big-city police departments, according to a Scripps Howard News Service study of crime records provided by the FBI.

"This is very frightening," said Bill Hagmaier, executive director of the International Homicide Investigators Association and retired chief of the FBI's Behavioral Analysis Unit.

"We'd expect that — with more police officers, more scientific tools like DNA analysis and more computerized records — we'd be clearing more homicides now with more resources," Hagmaier said. "But the clearance rates have fallen drastically."

Nearly 185,000 killings went unsolved from 1980 to 2008, the Scripps study found.

Additionally, a survey of 1,001 adults interviewed by telephone from Feb. 3 to March 9 by Scripps Howard News Service and Ohio University found nearly one-third of American adults personally know the victim of a homicide and at least one in nine personally knew the victim of an unsolved homicide.

The odds of knowing a homicide victim are greater among racial and ethnic minority groups, among men and among less-educated people, the poll showed.

The public is starting to notice the growing percentage of unsolved killings.

"When my first son was killed, I was embarrassed and ashamed. Why did this happen to me? But when my second son died, I decided I'd had enough and wanted to be an advocate for murder victims," said Valencia Mohammed, founder of Mothers of Unsolved Murders, in Washington, D.C.

Mohammed's 14-year-old son, Said, was found shot to death in his bedroom in March 1999. His elder brother, Imtiaz, 23, was shot to death along a city street in June 2004, prompting Mohammed, a former member of the D.C. Board of Education, to demand a meeting with top police officials.

"I asked, 'How many unsolved murders do you have?' They said 3,479 since 1969. That's when I broke down. I was in tears. I said, 'I know you guys are not going to solve these murders.' "

Police made an arrest and obtained a conviction four years after Imtiaz was killed, but Said's death has not been solved.

Experts say that murders have become tougher to solve because there are fewer crimes of passion, where the assailant is easier to identify, and more drug- and gang-related killings. Many police chiefs — especially in areas with skyrocketing numbers of unsolved crimes — blame a lack of cooperation by witnesses and even surviving victims of violent crime.

Still, some police departments routinely solve most of their homicides, even the tough ones, while others are mired in growing stacks of unsolved cases.

Police solved only 35 percent of the murders in Chicago in 2008, 22 percent in New Orleans and just 21 percent in Detroit. Yet authorities solved 75 percent of the killings that same year in Philadelphia, 92 percent in Denver and 94 percent in San Diego.

"We've concluded that the major factor is the amount of resources police departments place on homicide clearances and the priority they give to homicide clearances," said University of Maryland criminologist Charles Wellford, who led a landmark study into how police can improve murder investigations.

Wellford served as a consultant in the Scripps study, which found enormous variation in murder-clearance rates around the nation. But the patterns are not random. Police departments that showed the most dramatic improvement made concerted and conscious efforts to do so.

After clearance rates in Philadelphia dropped to an anemic 56 percent in 2006, newly elected Mayor Michael Nutter declared a "crime emergency."

He hired Charles Ramsey, a former police chief in Washington, D.C., as police commissioner. Ramsey installed a fresh homicide supervisor, Capt. James Clark, who led a results-based oversight of murder investigations similar to total-quality management methods first employed by Japanese manufacturers.

"This is just like in any industry," said Deputy Commissioner Richard Ross, a veteran Philadelphia homicide investigator and major-case supervisor. "If you don't work a job, then it's not coming in. That's the saying around here. So we make our guys work the jobs."

Clearance rates jumped to 75 percent in 2008.

The turnaround of solution rates in Philadelphia has been repeated in dozens of police departments around the nation, the Scripps study found.

"If police organizations say it's unacceptable to have clearance rates of 50, 40 even 30 percent, then those rates will rise," Wellford said. "They begin to institute smart policing in their homicide investigations."

The nation's most dramatic improvement, according to the Scripps study, was in Durham, N.C., where clearances averaged only 39 percent in the 1990s following a dramatic increase in drug-related crime. But the solution rate shot up to an average of 78 percent for the city's 215 killings since 2000.

"This doesn't happen in a vacuum," said Durham Police Chief Jose Lopez.

Durham's department uses several best practices espoused by criminologists and Justice Department researchers:

Generous authorization of overtime for murder investigators during the critical first hours after a killing.

More manpower at the murder scene, including up to eight experienced detectives.

Sharing electronic information between homicide investigators and the intelligence and crime-analysis units.

Sending forensic-science technicians to crime scenes, and expanding in-house lab procedures for fingerprint and ballistics analysis.

In addition, they developed Durham's Community Response Program to locate witnesses after a major crime.

"We will canvass door-to-door to see what information we can get. If necessary, we'll get up to 100 officers knocking on doors," Lopez said. "It's civilians, police, even elected officials who come out so we can get more witnesses … witnesses we otherwise would never have gotten. And that builds more trust throughout the neighborhoods."

While several police departments have shown similar improvements, most have not. The average solution rate fell in 63 of the nation's 100 largest police departments.

City police in Flint, Mich., and Dayton, Ohio, suffered the worst decline in clearance rates among major police departments. The average clearance rate fell more than 30 percent since the 1990s in both cities.

"Often we know with some degree of certainty who committed homicides but do not have sufficient witness cooperation needed for proof beyond a reasonable doubt in court," said Dayton Police Chief Richard Biehl.

Both Biehl and Flint Police Chief Alvern Lock questioned the accuracy of the clearance data from the FBI. However, the police departments provided the data to the FBI, prompting Biehl to order a staff review of past homicide cases in Dayton.

Both cities suffered a substantial decline in manpower, the result of rapidly declining tax bases in their communities following severe cuts in manufacturing jobs. Since 1990, Flint dropped from 330 sworn officers to 185, while Dayton's force went from more than 500 to 394.

"I don't think you can totally lay this (declining clearances) to the number of sworn officers here. That's just part of the problem," said Lock. "Witnesses don't want to cooperate with police. And now, even the (surviving) victims don't want to cooperate."

But Lock said budget constraints have hurt.

"If I had a magic wand, I'd ask for more money so I could hire more officers. We just need more of everything," Lock said.

Contributing: Elizabeth Lucas

e-mail: hargrovet@shns.com