A federal grand jury here is probing allegations that a drug ring used a network of Capitol Hill aides to distribute high-quality cocaine to government offices there, according to sources familiar with the investigation.

A joint task force of federal and D.C. police investigators has received unsubstantiated charges from an alleged ring member now cooperating with authoritites that the ring's customers included congressmen and their aides, sources said.

The drugs allegedly were distributed by Capitol Hill aides who work at jobs such as tour guides, pages and on the staff of the House doorkeeper, sources said. The grand jury has recently issued subpoenas for some of the suspected couriers, sources said.

The informant, identified as Robert A. Finkel, 30, began cooperating with investigators after he and two other men, Troy M. Todd Jr., 22, and Douglas W. Marshall, 26, were arrested April 19 for allegedly selling cocaine to an undercover police officer at Marshall's parents' home in the District. Marshall, who lives at the family residence in Forest Hills, is the son of international lawyer Sylvan Marshall, who has represented several foreign embassies.

His mother, who was in her bedroom reading when police made the arrest in another part of the house, has been subpoenaed to the grand jury, sources said.

A woman who answered the phone at Sylvan Marshall's law office yesterday and identified herself as an assistant to Marshall said he was unavailable for comment. "I can tell you right now that he's not going to comment," she said.

According to an arrest affidavit filed in U.S. District Court, Marshall, Finkel and Todd were arrested after Marshall handed eight ounces of cocaine to an undercover narcotics detective in exchange for $15,000. The detective then signaled to other officers who rushed into the house and made the arrest.

All charges against the three men were later dropped and the matter is now being presented to the grand jury as part of a larger investigation into the alleged ring's activities, sources said. Finkel, who has already testified before the grand jury, is in the protective custody of law enforcement officials, sources said.

Sources familiar with the probe said that police and U.S. Drug Enforcement Administration agents are upset because they feel that prosecutors in the U.S. Attorney's office here are reluctant to pursue allegations that public officials used drugs supplied by the ring.

Prosecutors in U.S. District Court usually focus on major drug distributors and not users, but in this case investigators believe that since some of the alleged users might be elected officials the probe should be allowed to include them, sources said.

One law enforcement source described the alleged operation as "probably the major quality cocaine ring operating in the D.C. metropolitan area."

During discussions leading to the arrests on April 19, Marshall told the undercover agent that he would sell him 10 ounces of cocaine for $2,000 an ounce and that it would be 85 percent pure, according to the arrest affidavit.

Marshall told the agent he would have to get the drugs from another location, the affidavit said.

When Marshall returned he allegedly sold the eight ounces of cocaine to the agent, according to the affidavit.