Nearly 100 of the killings in Washington this year remain a mystery to D.C. police, who say the unprecedented number of unsolved cases illustrates rampant drug violence on city streets and the increasing burdens placed on homicide detectives.

The 227 slayings committed in the District in 1987, 29 of which have occurred this month, are the highest number recorded since 1975, when 235 were reported. About 60 percent of the killings this year have been linked to the city's flourishing drug trade, which has made investigating slayings significantly more difficult, police say. Most of the unsolved homicides are drug-related, police said.

In 1986, less than one-third of killings were drug-related, according to police statistics, and drugs had a role in less than one-fourth of the 148 homicides -- the fewest in 20 years -- recorded in 1985.

"A vast majority of the cases still open are the ones tied somehow to drugs," said Capt. Larry Soulsby, who commands the police department's homicide division. "And the closure rate is directly affected by that."

Through the first six months of 1987, D.C. police reported 101 homicides to the FBI. Washington ranked second among U.S. cities in per capita slayings during that period, according to calculations based on FBI statistics. Detroit, ranked first in per capita homicides, reported 308 killings in the first six months of this year. Baltimore, ranked fourth per capita, reported 110.

Soulsby, other police officials and detectives said in interviews this week that the sudden rise in killings resulting from drug deals or drug battles often has made investigations nearly impossible. Slayings have been committed in past years over drugs, police said, but never before has such competition existed among well-armed drug traders. The fight to control city drug markets has led to escalating violence, police say.

Detectives called the number of open cases this year "astounding" and "pathetic." In 1986, 44 of the 197 killings were unsolved at the end of the year.

"There's something really wrong when there's this many cases open," said a veteran detective. Department policies forbid detectives to be quoted by name. Said another detective: "To be close to 100 open cases is very high. It should be half that. If you look at the numbers, the chances of getting away with murder is getting close to 50-50. That's real bad."

Police attribute the number of open cases to the lack of cooperation from witnesses and the transience of well-armed drug dealers from Baltimore and New York who travel to Washington under false names, sell drugs and leave.

Drug "hits," as police call them, have become commonplace; in the past two weeks, two men, one from New York, were slain, apparently over drugs, and were found lying in alleys in Northeast and Southwest Washington. One man had been shot nine times, the other 12.

In past years, detectives rarely had to leave town to pursue suspects, said Lt. Charles Bailey of the homicide division. "But this year, numerous times we had to send our guys to New York to look for clues," Bailey said. "What's happening is this: Guys come into town, make a kill, and they're out on the next shuttle and no one's ever seen them before."

Detectives also contend that gaining witnesses' cooperation always has been difficult, but was never the obstacle it has been this year. Often, potential witnesses also are involved in drug trading, detectives said, or are frightened by drug dealers who control the drug trade on a block or neighborhood.

"Even the law-abiding citizens who can't stand drugs in their neighborhood are so fearful of drug dealers that they don't even want to be seen near police, much less talk to them," Bailey said. "We constantly get 'I just can't talk to you out here' at the scene of a murder. They know the dealers are watching who speaks."

Police say the problem is most significant in the department's 7th District, bounded by Pennsylvania and Southern avenues and the Anacostia River in Southeast Washington, where they must respond to violence in some of the city's busiest drug markets. According to police statistics through mid-December, 48 of the 62 killings within 7th District boundaries had roots in drug trading.

After a shooting death two weeks ago in Southeast that police believe may have resulted from drugs, many neighbors avoided police officers and detectives on the scene. Some residents said the block where the killing occurred is infested with round-the-clock drug sales.

One man, who declined to give his name for fear of reprisal, pointed to faces in a crowd watching police outside the victim's home. "Those guys deal. And look at them just watching this." The man said the dealers were eager to see who cooperated with police.

"We know they watch," said one detective. "It's frustrating how they scare a neighborhood."

Between 1970 and 1980, police closed 82.5 percent of homicides, according to the department's annual reports. The rate of closed cases each year for the first half of the 1980s was 70 percent. This year, with about 87 cases open and 10 others with warrants but no arrests, the percentage of closed cases is slightly above 60 percent.

"The public perception used to be 'You can't get away with murder, right?' " said Gary Hankins, chairman of the Fraternal Order of Police in the District. "Well now, it seems like almost half the time you do. That's a damn scary thing."

Hankins, as well as some homicide detectives interviewed, suggested that other problems, in addition to the rise in drug battles, have contributed to the number of unsolved cases. Chief among those, they said, is sinking morale and building resentment within the homicide division's 44-member squad.

"Every night, there are so many shootings that you have to go to, and have to put aside a case you were working on, which can get cold real quick," a detective said. "I've had to go to three or four incidents on one shift a bunch of times this year. How do you work on all of them at once?"

In addition, Hankins and others contend that detectives resent the police department's Operation Clean Sweep, an aggressive undercover effort to quash street drug sales that has resulted in more than 20,000 arrests since it began in August 1986. They suggest Clean Sweep money would be better spent on paying detectives overtime to solve cases. "I've heard the same scenarios over and over from detectives," Hankins said. "It's 'You can't work over eight hours. Go home.' "

Other police officials dispute Hankins' point. "We can't lose sight of the fact that street drug sales are a driving force behind many of the murders," said Capt. William White III, a department spokesman. "Clean Sweep operations often gather intelligence that homicide can use."

Some homicide detectives charged that infighting also has contributed to declining morale, suggesting that as the number of open cases rises, so does the pressure from superiors to solve them. Instead of such pressure leading to healthy competition, detectives said, it has prompted reluctance to help out one another. Three detectives said some of their coworkers deliberately do not answer radio calls for serious shootings in Southeast.

Soulsby admitted that the division has morale problems, but said they are caused by the numbers and kinds of homicides this year. Even in the early 1970s, when record numbers of killings were committed in Washington, most cases were solved quickly because more often than not the violence was the result of domestic disputes, Soulsby said.

"The biggest impact on morale is the amount of time worked by detectives," Soulsby said. "They're in court at least three times a week, then trying to follow cases they've already been assigned, then may have to report to a shift where new cases come up. And when you're doing that week in and week out, of course that affects morale."