A D.C. police sergeant was shot to death by a robbery suspect on the bus platform of the Minnesota Avenue Metrorail station last night, seconds after the policeman had escorted the man off a crowded Metro shuttle bus, District police and Metro officials reported.

Sgt. Joseph M. Cournoyer, 30, who was assigned to the 6th Police District, was shot once in the chest at close range about 7:45 p.m. by the gunman, apparently one of two men who minutes before robbed a Murry's Steaks store less than a block away, police said.

Cournoyer, who had been on the police force six years and was promoted to sergeant six months ago, was pronounced dead at 8:48 p.m. at Prince George's General Hospital, police said.

Late last night and early today, powerful lights illuminated the neighborhood as scores of police officers, assisted by helicopters and tracking dogs, worked to seal off an area stretching from Woodson Junior High School at 4101 Minnesota Ave. NE, east and north along Grant and Hayes streets as the search for the gunman and the second robbery suspect continued.

Police Chief Maurice T. Turner Jr. said at a news conference last night that Cournoyer was not wearing a bulletproof vest when he was shot, adding, "I can't answer why he was not wearing a vest."

Police officials said all regular patrol officers had been issued bulletproof vests, purchased with private contributions to a massive campaign initiated after two D.C. policemen were shot at close range and killed in the fall of 1982.

Those two officers, Donald G. Luning, who was killed Sept. 14, 1982, and Robert K. Best, who was shot to death Dec. 15, 1982, were the most recent District police officers to be killed while on duty.

Luning was shot to death with his own service revolver as he struggled with a stolen-car suspect in a building at 3694 Hayes St. NE, five blocks from where Cournoyer was fatally shot last night.

It was unclear last night if Cournoyer had been issued a vest, officials said. Assistant Chief Isaac Fulwood said wearing the vest, which can deflect a bullet or lessen its impact, is not mandatory.

Police said that about 10 persons and a driver were on the bus when Cournoyer confronted the suspect, but that none was injured.

Turner said last night that as an officer, Cournoyer "apparently had something on the ball," and praised the officer's alertness in spotting the robbery suspect on the waiting bus. But Turner and other department officials could offer few details about the robbery and fatal shooting.

According to information provided by them, authorities at the scene, neighborhood residents and Metro officials, the events leading to Cournoyer's death began shortly before the 8 p.m. closing time at Murry's Steaks Inc., located at 4061 Minnesota Ave. NE in the Park and Shop shopping center between Benning Road and Grant Street NE.

Police said two men, one armed with a pistol, entered the store, demanded money and fled with an undetermined amount of cash north toward the Metro station.

Several employes, but no customers, were in the store at the time, but no shots were fired and no one was injured, police said.

Cournoyer, on duty alone in a marked police cruiser nearby, apparently heard a radioed report of the robbery and went to the area to investigate, officials said.

Cournoyer, who was in uniform, boarded the waiting Metro shuttle bus, according to police Inspector Richard Pennington, spotted a passenger who fit the description of one of the holdup men and asked him to stand up. Pennington said Cournoyer performed a brief "pat-down" search on the man, then asked him to step from the bus.

Cournoyer and the man left the bus by the front door and were standing a few feet away at an information kiosk on the loading platform when they became involved in a scuffle, officials said. During the struggle, officials said, the man pulled a pistol from under his jacket and fired one shot, striking Cournoyer in the upper chest near his heart. Cournoyer never drew his revolver, Turner said.

The gunman ran north along Minnesota Avenue, and may then have fled eastward along Grant or Hayes streets or Gault Place, police said. It was unclear last night how the second robbery suspect made his escape. No arrests had been made as of early today, but police investigators said shortly after 2 a.m. that they were questioning a possible suspect.

Several residents near the Minnesota Avenue station said they did not hear the shooting, but were alerted to the incident by the sirens of police cars pouring into the neighborhood.

"I knew something terrible had happened because of all the police cars, like last week," said one elderly woman, referring to an incident last Saturday at the station in which a 2-year-old girl was strangled when a tie string on her jacket became caught in an escalator.

Cournoyer's wife, Darlene, and his brother-in-law arrived at the hospital shortly after being told of the shooting. Also gathered outside the operating room where doctors worked to save the officer were numerous police officials and police chaplain the Rev. Joseph Dooley.

Police officials said Cournoyer and his wife lived in Riverdale. The couple had no children.

Officer Richard Bray, who was Cournoyer's training partner when Cournoyer was assigned previously to the 1st District, said in a telephone interview last night that Cournoyer was from Rhode Island.

"He was a very good police officer," Bray said, the kind who"didn't stop being a police officer just because he got promoted. That's how this thing happened tonight."