Two suspects were arrested yesterday in the slaying of D.C. Police Sgt. Joseph M. Cournoyer at a bus stop Tuesday, as scores of heavily armed police descended on a Northeast apartment complex before dawn, climaxing a 31-hour manhunt.

The arrested men -- Sylvester R. King Jr., 35, of 3697 Jay St. NE, and Charles A. Blackwell, 27, of 3693 Jay St. NE -- were both charged with felony murder in Cournoyer's death and with the armed robbery of a meat store shortly before the police sergeant was shot.

Both were being held at D.C. Jail pending a hearing Tuesday on whether they should be held without bail until trial. U.S. Attorney Joseph E. diGenova said a third suspect remained at large, and police sources confirmed that the third person was believed to have served as a lookout during the armed robbery.

Police officers wielding sledgehammers broke into King's second-floor apartment and arrested him around 3 a.m. About 30 minutes later, officers moved to an apartment two doors away, broke through the door, and arrested Blackwell.

According to documents filed in court, King was on parole and Blackwell was on probation for previous convictions at the time of the shooting. King had previous convictions for grand larceny, petty larceny, unauthorized use of a motor vehicle, destruction of property, uttering and burglary.

According to the documents and an interview with his mother, King is a former heroin addict whose parole from the federal penitentiary in Petersburg, Va., was scheduled to expire in 1989.

Blackwell, who moved here from Norfolk in July, was previously convicted of attempted robbery and petty larceny, according to court records.

Their arrests, which came after "hundreds of interviews" and a few "key tips," according to a homicide investigator, followed an intense manhunt in the area of the Minnesota Avenue Metrorail station, where Cournoyer was fatally shot at a bus platform while attempting to apprehend a robbery suspect.

Cournoyer, a six-year veteran who was on duty in a marked police cruiser nearby at the time of the robbery, responded to a radio report of the holdup at a Murry's Steaks store about 7:45 p.m., and drove to the Minnesota Avenue Metrorail station -- less than a block away from the meat store -- to search for suspects, police said.

There, Cournoyer boarded a waiting shuttle bus, where he spotted a passenger matching the description of one of the holdup men, police said. Cournoyer searched the man and then escorted him off the bus, when a fight broke out, according to police.

During the struggle, the passenger -- identified in the police affidavit as King -- pulled an automatic handgun, shot Cournoyer once in the chest and fled.

An ambulance transported Cournoyer to Prince George's County Hospital, where he was pronounced dead about an hour later.

According to the police affidavit, King was identified as the assailant by a witness who reported knowing him and watching him board the bus and later shoot the police sergeant.

A separate affidavit filed in the case against Blackwell said that he told police he participated in the robbery.

Police sources said that Blackwell and the lookout man who remains at large were apparently near the bus when Cournoyer boarded it and ordered the robbery suspect outside.

King's mother, Evelin King, 64, who shared the Jay Street apartment with her son, said yesterday, "I don't think it was my son because he is not a violent man."

Her son "has had a drug problem for quite a while," she said, but she added that he seemed to have stopped using drugs about four years ago.

Evelin King said she was not at home when her son was arrested because she was "upset" by the many police who had begun congregating in the neighborhood earlier in the day. She said she went to stay with a friend.

She said her son called her about midnight, three hours before his arrest, and she warned him not to go outside because he has a light-colored complexion, as did the suspect being sought by police.

"He said, 'Mamma, I'm scared.' I said, 'don't go out. . . 'I said 'don't go out because somebody will kill you.' "

Mrs. King said that after police told her that her son had been arrested, she returned home and found her door broken in, her bed stripped and her closet emptied.

"They police turned and shook it up pretty well. Everything was on the floor . . . . They didn't have to tear up my door and tear up my house, but I know what they were doing," she said. "They were looking for a gun or some money."

As well as being neighbors, Blackwell and Sylvester King Jr. worked for the same construction company, Miller and Long in Bethesda, where both men were laborers, according to Mrs. King and bail agency papers filed in court. A company official declined to comment.

Mrs. King said that she talked to homicide detectives yesterday after her son's arrest. She said the detectives told her that they had received two calls that had proven to be the biggest tips in the case, one from a woman, identifying King , as the man police were looking for.

Police were offering a $7,500 reward for information leading to the arrest and conviction of the gunman in Tuesday's shooting.

"I asked them the police , 'please don't hurt my child,' " Mrs. King said. She said she was allowed to talk to her son, "who told me that they hadn't done anything bodily to him but that he was scared."

"Since 1980, he was not on drugs," she said. "He was clean, he had a good job and his act was together. When he got out of jail, he said 'Mamma, when I come home I'm never going to jail again because I hurt you too much.' " Staff writers Lyle V. Harris and Joseph E. Bouchard contributed to this report.