For D.C. police homicide detectives, it was a typical "mystery murder," meaning that when they arrived at the apartment in Northeast Washington last week where a man lay nude on the floor with a bullet in his head, there was no one around with a smoking gun.

The shooting death of William David Nicol, a 36-year-old civilian analyst for the U.S. Coast Guard, appeared to be gay-related, among the most difficult murders to solve because of the secrecy that often surrounds their lives, police said. Last year, police solved l42 of the 205 murder cases they investigated. But of the 14 involving men believed to be gay, only three cases were closed.

This year, however, an arrest was made in the case of Willam Nicol within 48 hours. In fact, all but two of the l0 murder cases investigated by D.C. homicide detectives this year have been solved and arrests in the unsolved ones are "imminent," according to Capt. Jimmy L. Wilson, head of the Metropolitan Police Department homicide branch.

Wilson attributes the success to a new police procedure he started last year in which squads of homicide detectives team up with uniformed officers from various police districts and sometimes even classes of as many as 100 cadets from the police academy to saturate a neighborhood where a murder has occurred.

"We assemble everybody in the location where the crime has occurred, and give the younger members printed questions to ask," Wilson said. "We knock on every door in a neighborhood and return if no one answers. We keep the pressure on. We have found out that there is information out there in the community, but if you don't go get it, it won't come to you."

Using the new techniques, teams of detectives combed the crime scene looking for clues in the Nicol slaying. During a search of Nicol's apartment in the 500 block of Ninth Street NE, investigators discovered evidence that they said indicated that Nicol had been visited by someone from Williamsburg, Va.

Once in Virginia, detectives picked up a new trail that led to Richmond, where, they said, they found Nicol's car. The trail ended, police said, at a hospital in suburban Henrico County with the arrest of an 18-year-old Richmond man who had recently checked in there. He was charged with homicide, and is awaiting extradition to the District of Columbia.

Police consider a case closed at the time of arrest, regardless of the eventual disposition of the case in court. Wilson said yesterday that he expects convictions in 98 percent of his cases.

The change in homicide procedures underlies a deeper change within the Metropolitan Police Department, which began in l979 when Deputy Chief Alphonso Gibson, former head of the department's internal affairs divison, replaced William C. Trussell as head of the Criminal Investigations Division, which includes the department's robbery, burglary and sex offense units as well as the homicide branch.

Under Trussell and previous CID commanders, the homicide unit routinely solved about 85 percent of its cases.

But, according to Wilson, who worked for Gibson in the internal affairs division, the homicide branch was too independent. "Nobody from the outside dared interfere . . . . My job was to bring the homicide branch back into the police department."

Today, most members of the homicide branch have less than five years experience in the unit.Some of the older detectives who complained that this inexperience was causing a decline in case closure rates have now retired or been reassigned to other units.

Wilson, 37, calls his new men the "Redskins of the police department." He says what they lack in experience, they make up with enthusiasm.

"We underwent a reorganization of the homicide unit and we had to replace a lot of veterans with some younger, more enthusiastic investigators," said Wilson, who took over the unit in l980. "Granted, we lost a lot of experience. So now we have to go out there and prove ourselves. Like the Redskins, we're working for respect."

The homicide branch investigates about 4,000 cases a year, including suicides, non-traffic accidental deaths, and natural death cases not attended by a physician.

"We have found that certain types of homicides are more offensive than others," Wilson said. "For instance, when you find two people out in the streets fighting over narcotics and one kills the other, the public doesn't get too excited. But when you have a young lady who is a fine person in the community and she gets raped and murdered, the community gets extremely upset and our phones jump off the hook night and day with information. That's how we solve a lot of cases."

For victims of crime in Washington's gay community, there is markedly less information volunteered. Because so many of the gay murders that have occurred in Washington during the past year are unsolved, members of that community have offered monetary rewards for information that may lead to the arrest of a suspect.

"What makes gay murders difficult to solve is that the people historically have lived in a closed society without mention of who their friends are or what time or where they're going to meet them," Wilson said. "When someone turns up dead, there is little to go on."

But, Wilson continued, "Every crime scene contains something that the criminal has left behind. We just have to find out what that is."