Autopsies on seven of eight victims in the weekend outbreak of drug overdoses here confirmed that heroin was the cause of death, and the Washington Hospital Center reported an additional drug fatality yesterday that may be related.

"We know they [the seven victims] died of a heroin overdose," said Rak Kim, acting chief of the D.C. medical examiner's office. "The amount of heroin was large enough to kill them."

D.C. police, meanwhile, continued to search the city for the dealer or dealers responsible for distributing the heroin, apparently an especially pure strain that has been dubbed "J.R." in street parlance. In the past, other particularly potent strains of heroin that have made their way to Washington neighborhoods have been nicknamed "Murder I," "Jordache," "Mainline" and "Dynamite," among others.

"We have no suspects," Assistant Police Chief Isaac Fulwood Jr. said yesterday at a news conference at police headquarters. He said the search was concentrated in Southeast Washington, where most of the fatal overdoses occurred. At least a dozen other persons suffered nonlethal heroin overdoses, apparently from the same powerful batch of heroin, and police investigators were also planning to interview all of these victims.

The latest fatality, Thomas Weems of 2765 Naylor Rd. SE, who was in his mid-30s, was admitted to the Hospital Center at 3:45 p.m. yesterday and died 35 minutes later, according to Mary Anderson, a Hospital Center spokeswoman. "We have reason to believe that Weems' death is connected to the other overdoses," she said.

Fulwood said police "are tying it the ninth in because they all occurred in a time span when potent drugs were available on the street."

Between 8 p.m. Friday and 10:55 a.m. Sunday eight people died of drug overdoses. Seven of the victims died in the District and one in Prince George's County. An unknown number of people who survived the overdoses were treated where they were found with Narcan, an antidote that temporarily erases the effects of an overdose, and then declined further medical attention.

Results of the autopsy on the Prince George's County victim, performed in Baltimore, have not been released.

Charles Seigel, a spokesman for the D.C. medical examiner's office, said two doctors and one toxicologist were brought in on an emergency basis late Friday to expedite the tests on the first victims.

D.C. police were still awaiting the results of those tests to determine whether the heroin sold to the victims was unusually potent or contained a harmful contaminant.

"The fact that they died of heroin does not mean that there was no foreign substance," Fulwood said.

One vice officer, who asked not to be named, said that whoever sold the heroin didn't know what he or she was doing. "Someone slipped up," he said. "No dealer would intentionally sell heroin of that quality. Why would they sell something at, say, 20 percent when they could cut it down to 5 percent and still sell it all off at the going rate of $30 or $40 a dose?"

Potency of a street drug, he added, is always a problem for users because there is no real test for street people to assess it. "The only way to test it is to shoot it into a junkie and see what happens," he said. "You usually can guess the quality of the drug by the price."

Assistant Police Chief Ronal Cox, who oversees the morals division, said, "We haven't gotten a sample yet and we have undercover officers out trying to make buys . . . . As you might expect, there is very little drug activity in that area now. The activity has just stopped at a lot of locations."