A 26-year-old man who police said was walking along a Mount Rainier street with a loaded shotgun partly hidden beneath his jacket was shot and fatally wounded Sunday night by an off-duty Landover Hills police officer who thought that he was a burglary suspect.

The man had nothing to do with the burglar alarm that brought Mount Rainier police to Party Time Liquors at 3307 Rhode Island Ave., police said, but he fled when officers arrived and he allegedly aimed the shotgun at the Landover Hills officer before the shots were fired.

The victim, Donald Ray Collins of 3703 36th St. in Mount Rainier, died yesterday after surgery at Prince George's General Hospital, county police said. Collins was the third person fatally wounded by area law enforcement officers since Thursday.

The Landover Hills officer, Richard Hartnett, fired three shots at Collins, striking him twice in the abdomen, police said. Hartnett, who was not in uniform, was aiding a uniformed Mount Rainier officer who had responded to the liquor store's alarm.

The Mount Rainier officer spotted Collins running away from the liquor store with what appeared to be the stock of a rifle protruding from the bottom of his waist-length jacket, according to county police spokesman Robert Law. The Mount Rainier officer left his car and ordered Collins to stop, Law said, but Collins continued to run south on Rhode Island Avenue.

Hartnett saw Collins running, identified himself as a police officer, and ordered Collins to halt, Law said. Collins stopped, pulled a shotgun from beneath his jacket and aimed it at Hartnett, Law said.

Collins was not involved in triggering the liquor store's burglar alarm, which police said was set off accidentally by store employes. Cpl. Bruce Gentile, a county police spokesman, said there had been no other burglary or robbery reports in the area.

"We're just not sure why he was running," Gentile said. "Of course, carrying a concealed deadly weapon is enough of a reason to run."

Collins lived in Mount Rainier with his parents and his brother and worked at the U.S. Treasury Department, according to the brother, Randy Collins, 24.

"I got mugged and beat up earlier in the night," Randy Collins, whose face was bruised, said yesterday. "He went out and did some things he shouldn't have done. He died of multiple gunshot wounds. That's all we want to say."

Hartnett, 26, has been placed on administrative leave with pay pending an investigation of the incident by Landover Hills Police Chief Carl Kaiser, according to Doris Nichols, the town administrator. She said that Hartnett had been a Landover Hills officer for about a month, and had several years' experience with the Riverdale and Cottage City police departments.

Collins was the fourth person to be shot by police in Prince George's County in 1985 and the second fatality. Three of the shootings and one fatality involved county officers, Law said.

During the same time, Montgomery County officers were involved in six shootings, one of them fatal. Fairfax County officers had two shootings and Arlington County officers had four, both without fatalities. Alexandria officers fired one shot in 1985, but it did not hit the suspect. District of Columbia officers, in the fiscal year that ended September 30, shot eight suspects, three of them fatally.

Two other recent fatal shootings also occurred in Maryland.

On Friday, a Capitol Heights man who police believed was a suspect in a drug sale, was fatally wounded by Prince George's County police after he appeared to reach into his trousers for a weapon as he was fleeing from police. No drugs or weapons were found on the man, Ronald Walker, 21, of 5711 Coolidge St.

On Thursday, a man who ran a red light near Waldorf and fled from police who tried to stop him was fatally shot, allegedly by a Maryland state trooper. Several shots were fired at the victim, James Harland Morris Sr., 29, of Bryantown, a rural community near Waldorf. State police said they were unable to determine whether Morris was armed or whether he fired at the troopers.

Most area police departments have similar guidelines concerning the use of deadly force. Generally, they specify that an officer can fire to defend himself or others when no apparent alternative exists for protecting himself or others from the threat of death or serious bodily injury.

Staff writer Gwen Ifill contributed to this report.