Washington homicide detectives yesterday discovered 4 1/2 pounds of heroin, with an estimated street sale value of $15.4 million, and $6,000 in cash in the small basement apartment of a gasoline station manager who had been fatally shot in the head by an unknown assailant.

Police narcotics experts said that the heroin, found in three cloth bags, is about 90 percent pure.

"This is the largest seizure of heroin in the department's history," said Lt. Carl Alexander at a press conference yesterday afternoon at police headquarters. The drug and the cash were discovered in a briefcase beneath a bed at 1929 Calvert St. NW.

"We expect this seizure to affect the heroin market in D.C.," Alexander said. "Our having it will mean that there will be a shortage out there on the street. The price of dope will probably go up. Crime may go up because junkies will need more money to pay the prices."

Police suggested that the dead man, identified as Rasheed K. Zaman, 34, of the Calvert Street address, was killed during a robbery or burglary. His body was found in a closet, police said. They said there were no signs of forced entry.

The assailant or assailants apparently ransacked the efficiency apartment looking for valuables, but found neither the heroin nor the money, police said.

"It wasn't hidden very well. If they the suspects would've searched the place properly, they would've found it," said Alexander.

The heroin and the briefcase containing the $6,000 were found while police were conducting a routine crime-scene search for evidence in Zaman's murder, which was discovered yesterday morning.

Police said the bed, a regular boxspring and mattress, was set so close to the floor that an inexperienced robber or burglar might not have thought there could be anything beneath it.

Zaman had left Washington last March on a month-long vacation to Pakistan, and recently spent a week in Chicago, according to his boss, Bob Cooper, co-owner of the Rock Creek Gulf station, located at 1827 Adams Mill Rd. NW in the heart of the Adams Morgan neighborhood.

Zaman told Cooper that "he had some business to take care of in Chicago" and had family matters to tend in Pakistan, where Zaman said he had family. "He said his mother was sick and he had to go visit her," Cooper said. "He showed me a telegram from his mother. He told me he had a wife and two kids in Pakistan he was trying to get over here. I just can't believe he was into dope. I figure someone like that wouldn't be worth nothing."

He said Zaman often worked seven days week, sometimes borrowed lunch money from coworkers and who recently sold an old Chevette -- a modest sub-compact-sized car -- and bought a used Pontiac Grand Prix.

Cooper said Zaman was "quiet, very friendly" and judged him to be the best worker at the busy gas station, where he made $5 an hour. "I'd like to have 10 men that work like that man did," Cooper said. "He was always doing something, always showed up to work early and stayed late."

Zaman, described by coworkers as 5-feet-5 and slightly built, had not been seen since Tuesday night, when he finished his evening shift at the gas station. Normally conscientious, he failed to report to work Wednesday afternoon for his 2 p.m.-to-10 p.m. shift, and a pair of his coworkers began to worry.

At about 7:30 a.m. yesterday, the coworkers went to Zaman's apartment, police said, and found Zaman's body crouched in a closet near the front door.

"People have been saying all day that it's just hard to believe all that money and dope was in his apartment," Cooper said. "He's the type of person who would let someone leave that stuff there, even if he knew it was illegal. He's that type of guy."

Police described their $15.4 million estimate of the street value of the heroin found in Zaman's apartment as conservative. The drug would have netted about $250,000 if sold wholesale, they said. Police said that the largest previous seizure of heroin as pure as that found yesterday was one pound.

Police said they have no suspects in the slaying.