Here are some of the reasons:
* Increasingly, victims are killed by people they don't know, often for petty and irrational reasons. Such ``stranger killings'' are much trickier to solve. With no clear pointers between killer and killed, it is hard for police to find suspects and establish motive.
* The average age of killers is falling. Youthful gunmen are more likely to kill impulsively and to kill strangers or casual acquaintances, leaving few clues.
* Civilians are less inclined to cooperate with police. A key reason is that it has become more common for murder defendants to threaten or kill people who might testify against them. The problem has grown so severe that Philadelphia District Attorney Lynne Abraham last week called for a municipal witness protection program, an unprecedented step for a local prosecutor. Police Commissioner Richard Neal has endorsed the idea.
* Because of cramped budgets and a rising murder rate, the typical Philadelphia detective has more killings to unravel. City homicide investigators complain privately that limits on overtime prevent them from staying on cases, even when the trail is hot.
``Not only are there more homicides to solve, but the nature of homicide is changing,'' said James Alan Fox, dean of the College of Criminal Justice at Northeastern University in Boston.
In the 1960s, Philadelphia police homicide detectives solved nine out of 10 ``jobs.'' Last year, when there were 433 slayings in Philadelphia, police solved 66 percent, according to police statistics.
The trend is national. In 1965, police departments across the country identified a suspect in 91 percent of killings that year. Nationally, police now solve about 65 percent.
The picture is even grimmer in big cities. In 1994, the nation's nine largest cities ``cleared'' 51 percent of all homicides. Homicides are cleared when an arrest is made or a suspect is otherwise identified, even if no conviction results. In Philadelphia, investigators fared slightly better: The clearance rate that year was 67 percent.
The new face of murder was evident in the case of Kiwanna Taylor.
The diminutive 18-year-old was killed last year in South Philadelphia in a fusillade of bullets that police believe was meant for her fiance. The fiance, who was shot and seriously injured, named two men, ages 18 and 20, as her killers in hospital interviews with detectives. He later recanted on the witness stand.
Charges against the suspects were dropped two weeks ago.
* Although erratic year to year, the overall clearance rate in Philadelphia has had a downward trend.
The downward spiral is driven, as Neal, Abraham and criminologists point out, by a growing reluctance of urban witnesses to cooperate.
And while it is still rare for witnesses to be killed in Philadelphia, it has happened in recent years.
There was the case of the dress-wearing and bewigged Donyell Paddy, who killed LaShawn Whaley, 20, in 1993 to stop her from testifying that Paddy and an accomplice had gunned down two men in North Philadelphia. Paddy has been sentenced to die for Whaley's murder.
And while no one died, a brazen shootout during a basketball game inside a Southwest Philadelphia recreation center three days before Christmas in 1993 demonstrated how hard it has become to get witnesses to help police.
Two young men pulled guns on each other inside the center's gym. Five people were wounded, including two of the players.
More than 100 spectators were in the gym. Not a single witness voluntarily came forward to testify.
Rec center volunteer Eddie Magic explained later, ``being that nobody got killed or anything, they just wanted to go on with their lives and not be looked upon as a dime-dropper or a snitch.''
* In the old days, said Chief Inspector Vincent DeBlasis, homicides almost came gift-wrapped.
A lot of them were what the police call ``mommy-poppy jobs,'' in which a husband or wife permanently decided an argument. Or, if someone was killed in a neighborhood, virtually everyone knew him. And they also knew who was mad at him.
``It's not like that anymore,'' said DeBlasis, the chief of detectives. ``We've become a city of strangers.''
That hurts a homicide unit that solves an estimated 80 percent of its crimes through witness identification.
The boom in crack cocaine in the late 1980s, drug-trade turf fights among youthful warriors, the continuing spread of handguns all have complicated the detectives' lot.
The killers have become more sophisticated, fleeing to Jamaica or Puerto Rico, making it difficult for police to find them.
``[The homicides are] much more complicated now, with drugs,'' DeBlasis said. ``We solve most of our homicides through witnesses, and when they don't come forward, it makes our job harder.''
Fox, one of the nation's leading murder researchers, said detectives everywhere were being overwhelmed by the sheer numbers of murders in America over the last few decades.
And more and more killers are younger and murdering strangers, Fox said. ``There are clear differences when you compare who kids kill versus who adults kill,'' said Fox. Younger murderers ``kill their friends, strangers, people they know only casually.''
Fox analyzed national murder data by separating killers into three age groups - teens, young men and mature adults - and found something surprising.
It turned out that all three groups were selecting victims about the same way as they did 25 years ago. That is, teens and young men continued to kill people unrelated to them, and mature adults tended to be more involved in domestic murders and the like.
What is different is that there are many more younger killers. (There were 48 teen killers in Philadelphia in 1994, versus 10 a decade earlier.) And with more younger killers, the total number of stranger victims has gone up nationwide.
Deputy District Attorney Charles Gallagher, who heads the district attorney's homicide-prosecution unit, said many of today's murders are not really random. Though they may get listed in the statistics as stranger killings, he said, ``there was a motive behind the killing - the drug trade.''
And once charges are brought, a heavy majority of cases end in convictions. Figures for the city's Common Pleas Court for 1992, the most recent available, show that 85 percent of murder cases end in conviction.
* In the Roundhouse quarters of the Homicide Division, a cramped first-floor office where the coffee is weak and the cynicism strong, there is worry over the growing number of unsolved cases.
For many of the detectives there, homicide is their life. Investigations are the axis of their friendships and family life and their sense of self-worth. When a case goes unsolved, it bothers them.
And, said one detective, the reason more cases are going unsolved is relatively simple: money.
``This is dollars and cents, that's what this comes down to,'' said the investigator, who requested anonymity for fear of reprisals from his superiors. ``The drop is because they have just clamped us down.''
As recently as four years ago, he said, the belief was that detectives should work whatever hours were necessary to solve their cases. Eighteen- and 20-hour days were not uncommon. The overtime helped some detectives earn more than $100,000 a year.
Now that's changed, the detective said.
The commanders want as much work as possible completed during regular shifts, when detectives are paid straight time. Sometimes, detectives are told at shift's end to wait to interview a witness until the next day, by which time the witness may have fled or developed a foggy memory.
``This is not a job where you say, `I'm going to wait until tomorrow,' '' the investigator said.
Still, he believes his fellow detectives are working harder than ever, and police statistics obtained by The Inquirer support him. In 1983, the average detective handled 4.7 homicides per year. By 1993, that caseload had more than doubled, to 8.9 killings per investigator.
Police Commissioner Neal said anyone suggesting that the department is not devoting enough money, manpower or energy to solving homicides ``is categorically wrong.''
He beefed up the homicide squad last fall after pulling some investigators out in 1992.
``We are making the overtime available to the investigators,'' Neal said. ``But as managers, it's not like you're going to say to somebody, `Go out and earn all the overtime you want.' ''
* An arrest can make an enormous difference for the loved ones of a murder victim, said Deborah Spungen, an area resident who founded Families of Murder Victims after her daughter, Nancy, was killed by English punk rocker Sid Vicious. It gives them a focus, an outlet for their anger.
Families also want to know how their loved ones died, believing knowledge can never be worse than imagination, though it sometimes is.
``People think when they find the murderer, or someone's arrested, that it's going to make it OK,'' said Spungen, ``and all that does is open another door. But I think people still would prefer knowing.''
George Thompson's son had drifted out of Roxborough High School before graduating but was trying to get himself together, his father said. He got a job at a restaurant and had just bought a car. He was working on a high school equivalency degree.
When Stephen Thompson drove to the Acme market in West Mount Airy the night of March 28, 1995, there was one available parking place, and he and another motorist wanted it. They began arguing.
When Thompson came out of the store, the other man followed him in his car. Not far from the store, the killer drew a gun and fired two shots. One hit Stephen Thompson's car. The other hit him in the head. He died the next day.
For Thompson, 47, who works for the city Department of Licenses and Inspections, the loss of his child is at times unbearable. Everything seems to remind him of Stephen.
Thompson is a deacon at St. Luke's Episcopal Church on Germantown Avenue but hasn't been as active since his son was murdered.
``It's thrown off my mind,'' he said.
He's unhappy with the police. They send out dragnets to find the killers of police officers, he said, but nobody bothers to return his phone calls about his son.
``Every time I call them, they say the detective is on the later shift, and when I call the later shift, they say he's on the early shift,'' Thompson said. ``There's nothing that will bring my son back. He's dead. But the fact that police have put the case on the shelf, and nothing's being done . . .''
Thompson thinks an arrest would help him deal with this loss.
``That,'' he said, ``would sort of put a cap on it.''