Tony Martinez Kelsey was a dedicated cop, his friends said yesterday, so dedicated that even while off duty he would cruise the streets in his patrol car and respond to radio calls.

But Saturday night, the 22-year-old Prince George's County policeman's dedication led to his death.

It was shortly after 9 p.m. when a couple walked into Cox's Liquors in Landover, and the man, either by mistake or design, displayed what appeared to be a bag of marijuana. It was an offense that many police officers routinely ignore. But Kelsey, who worked intermittently at the store as a security guard in off-duty hours, would not or could not ignore it.

Apparently, according to the store cashier, the man knew Kesley was an off-duty policeman. He held the bag of marijuana up to the young security guard, taunting him with the demand, "What are you going to do about it?"

While the man's female companion began shoving him out the door, Kelsey rushed from behind the counter. With his police radio, he called the Seat Pleasant police headquarters and said he was pursuing a man on foot. Those were Tony Kelsey's last words.

Minutes later, a policeman in a cruiser who responded to Kelsey's call found him sprawled on a patch of grass near the store.Kelsey had been shot in the head with his own off-duty gun.

Two hours later, Kelsey died in Price George's County General Hospital.

Acting Police Chief Jack McHale yesterday called Kelsey's death a tragedy. "And what really made it unfortunate," McHale said, "is that here's a guy who was known throughout the department as always doing a little more than he had to do. And that's why he got killed, for doing a little more than he had to do."

"Being a new officer, he was one of the more enthusiastic ones," said Sgt. Warren Jackson, one of Kelsey's earlier supervisors on the force. "He always gave one hundred percent. He lived and breathed the police department."

Kelsey's friends and his uncle said they did not know how long the young policeman had worked as a guard in the liquor store. Police in Prince George's County, unlike those in the District, are permitted to hold off-duty jobs as security guards.

"Tony wanted to be a policeman since he was in grammar school," said his uncle, Jack Brown of Southeast Washington.

In Prince George's County, where Kelsey joined the force, there has been a history of tense relations between black citizens and the predominantly white police department.

"He wanted to work in Prince George's County even though he's black because he thought he could do some good there," Brown said. "He worked real hard."

Kelsey was born in the District and graduated from Calvin Coolidge High School. He joined the Prince George's Police Department in 1978, and lived alone in an apartment in Hillside, Md., four miles from his precinct headquarters.

The police officer who was spending the day with Kelsey's mother, at her Southeast Washington apartment, said she was much too upset to come to the phone.

Prince George's County police yesterday released a composite sketch of the suspect, described as a black male, 27 years old with a thick mustache. They said the suspect may be armed with Kelsey's automatic pistol, a 9mm Astra -- taken from the security guard during the apparent scuffle outside the store.

Police are offering a $1,000 reward for information leading to the arrest and indictment of Kelsey's slayer.

The young police officer's friend on the force said yesterday they were not surprised that Kelsey chased the man with the marijuana.

"When you're a policeman and you see a law broken, you've got to do something about it," said fellow officer William Edge. "Tony felt strongly about that."