Federal prosecutors have begun dropping pending drug charges in cases involving D.C. narcotics officers who are the subjects of an FBI investigation into allegations that some officers in D.C.'s 4th Police District kept drugs and money confiscated in raids.

An assistant U.S. attorney told D.C. Superior Court Commissioner John W. King yesterday that two cases scheduled for a preliminary hearing had been dropped by prosecutors. The arresting officer in both cases was Shelton D. Roberts, a 4th District officer who has been named in the FBI probe.

Sources said yesterday that another drug case involving 4th District officers was dismissed Monday and that as many as 200 drug cases may be in jeopardy because of the pending FBI investigation of four or five 4th District vice officers suspected of skimming cash or drugs from drug raids.

U.S. Attorney Joseph E. diGenova said yesterday he had not yet decided what action to take in the drug cases or whether all 4th District narcotics cases will be dismissed, but that he expected to act in the next two days. DiGenova said he expects to discuss his plans with D.C. Police Chief Maurice T. Turner Jr. today.

Sources previously have stated that prosecutors may have to drop drug charges against defendants arrested by all 12 vice officers assigned to the 4th District because of possible legal challenges from lawyers who say that even if all the officers did not participate in the skimming they may have known about the allegations.

In the cases before King, one defendant was charged with distributing cocaine and another with possessing it. In both cases, search warrants were signed and executed by Roberts, a 17-year member of the police force, and other vice officers from the 4th District.

The prosecutor who announced the dismissals did not explain why the cases were being dropped and law enforcement sources differed yesterday on the reasons. Some said the cases were dropped because Roberts failed to show up to testify at the hearing, and pointed out that no decision had been made yet by diGenova.

The sources said that although it was highly likely that diGenova would order the dismissal of most cases involving Roberts and his partner, Officer James Whitaker Jr., who is also a subject of the federal probe, other charges may be reviewed on a case-by-case basis.

The investigation of the 4th District also includes allegations that some narcotics officers received payoffs from suspected drug dealers in exchange for warning of Operation Caribbean Cruise, an extensive drug sweep last year that netted few arrests. FBI agents searched the 4th District headquarters last week, reviewing search warrants and arrest reports.

William J. Garber, lawyer for one of the defendants in the dropped cases, said yesterday his client was not "at this point" making any allegation of missing property. "It's too early. We're still in the process of studying the matter," Garber said.

In other related court action, defense attorneys have seized upon uncertainty created by the FBI probe to seek continuances in drug cases. Judge Robert Scott granted a defense motion Monday for a one-month continuance in a case in which the lawyer cited the probe.