Scores of D.C. police officers, some equipped with flak jackets and armed with shotguns, combed through a 40-block section of far Northeast Washington yesterday but failed in their effort to capture the gunman who fatally shot a District police sergeant Tuesday night.

While squads of heavily of armed officers and dog handlers searched a number of buildings in response to telephoned tips, others went door to door circulating fliers and interviewing neighborhood residents.

"In the history of this department, there has never been a person who killed a police officer who hasn't been arrested," Assistant Chief Isaac Fulwood Jr. said of the vast manhunt. "We think we have a lead, but that's all I can say at this point. We do not have a suspect," he said at a late afternoon news conference.

"This really hits close to home," another police official said yesterday. "Everybody is aware of the dangers of the job, and the trouble is it gets to be routine. Something like this snaps everybody back into the real world . . . .

Fulwood yesterday released a composite drawing and description of the man who shot and killed 6th District Sgt. Joseph M. Cournoyer, 30, a six-year veteran of the force, at a bus stop outside the Minnesota Avenue Metrorail station about 7:45 p.m. Tuesday. Fulwood also announced that a $7,500 reward would be paid for information leading to the gunman's arrest and conviction.

Police said that moments before the shooting the gunman and an accomplice robbed a Murry's Steaks store less than a block from the Metro station. Cournoyer, who was in uniform and alone in a marked police cruiser, apparently heard a radio report of the robbery, went to the Metro station and boarded a waiting neighborhood shuttle bus to look for suspects, police said.

They said about 10 persons and the driver were on the bus, and that Cournoyer spotted a passenger who fit the description of one of the holdup men and asked him to stand.

Cournoyer did a "preliminary frisk" of the man, Fulwood said, and ordered him to the front of the bus and outside onto the platform. There, a fight broke out during which the man pulled a handgun from his jacket and shot Cournoyer once in the chest, Fulwood said.

He was taken to Prince George's General Hospital -- the closest to the shooting scene -- where he was pronounced dead about an hour later. An autopsy yesterday revealed that the bullet pierced Cournoyer's heart.

Cournoyer, who was not wearing the bulletproof vest he had been issued, apparently didn't draw his service revolver while on the bus because of concern for the passengers' safety, Fulwood said. He said that, while the investigation is under way, he was "not privileged to say what happened to the second man" involved in the holdup, and whether that man was on the bus at the time of the shooting or how he made his escape.

According to one police official, the department had received a large number of calls from people offering information, but that so far none had led them any closer to a suspect. The most dramatic moment of the search came about 5:30 p.m. yesterday when riot gear-equipped police surrounded an apartment building at 3693 Jay St. NE after an anonymous caller said one of the holdup men was inside. Police conducted a room-to-room search of the building but found nothing, officials said.

Officers throughout the department wore strips of black tape over their badges yesterday in public mourning for Cournoyer, who lived in Riverdale with his wife, Darlene.

Cournoyer, who was promoted to sergeant six months ago, was the 33rd D.C. officer to die in the line of duty since 1963. Twenty-two of those died of gunshot wounds. The last two D.C. officers slain on duty were Robert K. Best, who was killed in December 1982 in a fight with a gunman on Suitland Parkway, and Donald G. Luning, who was killed in September 1982 in a struggle with a stolen-car suspect five blocks from where Cournoyer was slain Tuesday. Both Best and Luning were shot to death, and neither was wearing a bulletproof vest.

Their deaths sparked a massive campaign that ultimately raised $634,000 to buy bulletproof vests for all D.C. police officers, and law enforcement officials said yesterday that Cournoyer's death was a grim reminder of the protective value of those vests.

"From what I hear, Cournoyer wore his vest all the time," said Sgt. Robert Hanbury of the 1st District, where Cournoyer was previously assigned. "He was a quiet individual and he was always standing erect, always sharp, shoes shined and neat as a pin. Of all the people not to have it on . . . . "

A police spokesman said that the department has issued about 3,000 vests to members of the department -- nearly everyone from the rank of sergeant down in the field operations division, which conducts routine street patrol.

Wearing the vests has not been made mandatory, he said, because of the large number of complaints made by rank-and-file officers that the vests are often uncomfortable.

Gary Hankins, spokesman for the Fraternal Order of Police, said yesterday that "the same quality that keeps bullets out keeps virtually 100 percent of the body heat in."

During the summer months, Hankins said, officers wearing the vests have had to be treated at the police and fire clinic for body rashes and heat fatigue caused by the vests, which weigh about 3 1/2 lbs. and are constructed of 10 layers of Kevlar, a man-made fiber that, pound for pound, has five times the strength of steel.

Vests worn by D.C. officers are designed to stop at .357 magnum bullet at close range, police said. Federal law enforcement officials said such vests are credited with saving the lives of more than 500 police officers since they were introduced nationwide in 1974. Currently, about 270,000 of the nation's 570,000 police officers have been issued protective vests, authorities said, but there are no figures available on how many are actually used on a day-to-day basis.

Funeral services for Cournoyer will be held at 11 a.m. Saturday at St. Bernard's Catholic Church in Riverdale, with burial to follow at Fort Lincoln Cemetery in Brentwood.

Police investigators asked yesterday that anyone with information about the shooting contact the D.C. police homicide division at 727-4347.