D.C. police and federal prosecutors yesterday launched a a major investigation to find the source of what they suspect is either a highly potent or contaminated batch of heroin that caused an outbreak of at least 20 overdoses, six fatal.

A seventh drug overdose victim was found last night in a house in Northeast, police said. They said no evidence immediately available linked the death to the others, but a syringe was found next to his body.

The U.S. attorney's office has begun to subpoena surviving overdose victims and other witnesses, including relatives, in hopes that they will testify before a grand jury and reveal information that could lead to the source of the drugs.

Police joined city leaders to urge heroin addicts to refrain from using the drug and seek help for their addiction at hospitals and drug rehabilitation centers.

The overdoses occurred between about 8 p.m. Friday and 3 a.m. yesterday.

Police have reason to believe that the heroin that caused the overdoses may be traced to a single source, because "four . . . deaths actually occurred during a one-hour span of time," Assistant Police Chief Isaac Fulwood said at a news conference.

Police said that they are not sure whether the heroin that caused the overdoses was unusually pure or was contaminated, because toxicology tests had not yet been completed. Tests confirmed, however, that high levels of heroin were found in the bodies of the victims who died.

Four overdose victims died between 7:45 p.m. and 8:45 p.m. and were found unconscious at separate locations, some in homes and one on a street corner, but all in the Anacostia area of Southeast and in Northwest Washington, Fulwood said.

A fifth District victim died later in the evening, Fulwood said. The sixth victim died later in the Maryland suburb of District Heights, he said.

Two of the surviving overdose victims apparently took the drug by inhalation, authorities said. The others injected the drug, according to police.

D.C. police identified four of the victims who died in the District as: Arthur Keeling, 28, of 4553 Akron St. in Temple Hills; Harold Ferguson, 29, of 845 Xenia St. SE; Henry Edwards, 34, of 1631 Euclid St. NW; and Deborah Claiborne, 32, of 425 Second St. NW.

Police said they were withholding the name of the fifth District victim until relatives could be notified.

Prince George's County police identified the Maryland heroin overdose victim as William Henry Purvey, 29, of 6886 Walker Mill Rd., Apt. 302, District Heights. He was found dead at his home.

County police declined to connect Purvey's death with the other overdose fatalities, although D.C. police did make the connection.

Last night's victim was identified by police as Jerry Thomas, 27, of 338 34th Pl. NE.

No arrests had been made in connection with the deaths as of last night, police said. Homicide detectives said that if a single source can be found, they will try to charge whomever is responsible with homicide.

Police complained that the overdoses are another dramatic sign of a continuing epidemic afflicting inner-city youths and young adults. All the victims were in their 20s or early 30s, all were black, and all but two were male, police said.

From 1981 to 1984 there were 424 narcotic-related deaths, most of them caused by heroin overdoses. The Friday night outbreak brings this year's total to 22.

"Drugs are very lucrative and, as long as there's money to be made, drugs will be sold. And I don't see anything out there that is keeping it out of the country," said police spokesman Lt. William White.

City Council member Wilhelmina J. Rolark (D-Ward 8), who represents far Southeast Washington, where police believe some of the potent heroin might have been sold and where about 10 of the overdoses occurred, appealed on television Friday night to addicts: "Please don't use drugs tonight."

Police Chief Maurice T. Turner Jr. said, however, that he feared addicts would do just the opposite.

"The sad thing is that in similar instances in the past, some drug users went right out and tried to find the drug that was killing the other users," he said. "They are seeking a high, and they are going to want that drug."

Mayor Marion Barry could not be reached for comment. Deputy Mayor Thomas Downs said:

"We have had a long run of drug overdose deaths for some time. This is unusual for the number of deaths in such a short time. Drugs are the most frustrating problem for this city. Heroin doesn't grow in the District. It is part of a national and international problem."