U.S. Attorney Joseph E. diGenova's pledge to file murder charges against those found responsible for selling the potent heroin that caused nine overdose deaths last week is based on legal principles that have not been tested in the District and have been explored only a few times elsewhere.

"There is a world of difference between showing that someone should have known the drugs he was dealing could kill someone, on the one hand, and showing that he actually did know," said William Mertens, a criminal law instructor at Georgetown University. "It's not well defined, but it's not without precedent, either."

The only known D.C. case in which suspected drug dealers were charged with murder was brought two years ago. Leonard McRae, the 37-year-old leader of a Southeast Washington drug ring, and two other men were charged with second-degree murder when an addict died after he injected himself with potent heroin distributed by McRae and two other men.

The murder charges against the two accomplices later were dropped. McRae pleaded guilty to a reduced charge of involuntary manslaughter and a charge of conspiracy to distribute drugs.

But the plea agreement included a provision that the sentences for the two crimes would run concurrently. McRae did not receive additional prison time as a result of the homicide conviction, and his attorney maintained there was "a questionable basis for a manslaughter conviction" in the case.

DiGenova insisted that "under the law it's clear that such a distribution of a deadly substance is in fact covered" by the city's murder statute.

"The key thing here is that we charged somebody with second-degree murder," he said. "As far as we're concerned, the law in that area is clear. There just isn't any question about the viability of such a charge."

DiGenova pledged last week to "pursue it vigorously in this investigation until we find who the murderers are, because that's what they are -- murderers."

He maintains that "supplying heroin of a deadly quality is the same thing as supplying a deadly poison."

But some attorneys would argue, as McRae's did, that the person who takes the drug has to assume the consequences. "They never would have been able to convict him [McRae] of murder," said attorney Sol Z. Rosen, who represented McRae in early stages of the case. "I would have loved to try that case."

According to the court record, McRae and more than a dozen others were arrested on drug distribution charges after a 10-month police investigation. McRae was charged with murder in the death of 27-year-old Jimmy Owens, a drug addict who collapsed after he injected himself with heroin while in the company of two of McRae's salesmen, who had sold it to him at a Southeast "shooting gallery."

McRae was not present when Owens took the heroin and, according to the operator of the "shooting gallery," one of the men charged with McRae had warned Owens the drug was unusually potent.

In addition, the autopsy performed by the D.C. medical examiner left unclear whether it was the drug specifically supplied by McRae that killed Owens.

Owens, the autopsy states, died of "acute and chronic (intravenous) narcotism."

McRae's attorney, Barry Simon, told the court during sentencing that "where the individual who receives the drugs is an adult in full possession of his senses and makes a voluntary and independent decision to use the drugs, it is at least debatable whether the person who provided him the drugs . . . can fairly be deemed responsible in the unlikely event that he suffers an overdose and dies."

There has been little research on the issue, and the few court rulings differ from state to state.

In 1969, the Supreme Court of Pennsylvania upheld the involuntary manslaughter convictions of a cigar store owner who had sold Sterno to alcoholics on skid row, 31 of whom died of methanol poisoning.

The court found that even though the men were negligent in drinking the Sterno, the store owner was more negligent because he knew they would drink the toxic liquid.

But in 1972, the New York State Supreme Court held that a man could not legally be indicted for manslaughter if he supplied a deadly dose of heroin. The law on homicide simply did not contain a provision for drug distribution, the court found.

In March 1983, the Supreme Court of Nevada upheld the second-degree murder conviction of a woman who gave pills containing trichloral ethanol to a young man who died. But the court held that the woman could be prosecuted for murder only because she helped the man take the pills. The question of dealers who sell drugs that are ingested away from their presence was one for the legislature to decide, the court ruled.

DiGenova cited a 1984 court ruling in Virginia upholding a second-degree murder conviction stemming from a deadly drug injection. But in that case, the murder charge was based on the underlying felony offense of drug distribution.

The charge of felony murder applies when a death results from the commission of another felony crime, such as robbery or rape. But the District is among several jurisdictions that do not include drug distribution as a crime for which felony murder applies.

The California courts have given a liberal interpretation of the rule, allowing convictions of second-degree murder even when drug distribution is not specifically contained in the felony murder statute.

But courts in other states, such as Kansas and Arizona, have rejected that idea in cases of overdose deaths, holding that the crimes that invoke a felony murder charge must be spelled out by the legislature.

The crimes for which felony murder can be charged in the District are listed by statute as arson, rape, mayhem, robbery, kidnaping and housebreaking while armed or using a dangerous weapon.

The case decided in November by the Virginia Supreme Court involved a man charged with helping give what the medical examiner determined was a single lethal injection of cocaine to a woman at a party in Chesterfield County four years ago.

Virginia law permits a second-degree felony murder charge when accidental death results from unspecified "felonious acts," and the court upheld the conviction of Forrest Perry Heacock on that basis in the first ruling of its kind there.

"I think this case stands for the proposition that when a person furnishes narcotics that cause death, he can be charged under the statute," said Senior Assistant Attorney General James Kulp.

Even when deaths are not involved, finding drug dealers can be a tedious process.

"These are extremely difficult cases to bring . . . " said one senior prosecutor here.