Federal prosecutors have asked judges in D.C. Superior Court and U.S. District Court to dismiss cases against 12 persons who had already been sentenced, citing questions raised about the cases as a result of the ongoing federal investigation of vice officers of the city's 4th Police District.

All of the cases involved 4th District officers now under investigation for allegedly skimming narcotics and money seized during drug raids. Some of the cases were brought as the result of searches conducted with warrants obtained using allegedly falsified affidavits.

U.S. Attorney Joseph E. diGenova announced Sept. 16 that his office, with the concurrence of D.C. Police Chief Maurice T. Turner Jr., would dismiss 300 to 400 pending drug cases "in order to preserve the integrity of the criminal justice system."

Sources said about 300 cases, which had not yet gone to trial, have been dismissed as a result of the police corruption investigation. These 12 cases are the first in which the government has sought to dismiss charges against defendants who had been convicted and sentenced.

Judges in the two courts have granted about half of the motions to dismiss the cases, and court records show that at least five persons, serving sentences of up to five years, have been released from custody as a result of the dismissals. One defendant, serving a sentence of up to 10 years, remains in jail because he has another case pending.

The cases involve cocaine, marijuana and PCP, ranging in amount from 78 milligrams of cocaine to more than 30 pounds of marijuana.

Several of the motions to dismiss, including one for a man sentenced to 30 to 90 months in prison, are still pending.

The motions filed in court do not give prosecutors' specific reasons for seeking the dismissals. A spokesman for the U.S. attorney's office said last night, "as promised last September, we have examined these cases on a case-by-case basis in order to preserve the integrity of the criminal justice system."

Most of the government's requests for dismissal involve defendants who were found guilty after a court trial that involved testimony from officers now under investigation. Four 4th District vice officers, including officers Shelton R. Roberts, Ronald C. James and James R. Whitaker, are believed to be the subjects of the federal probe.

However, D.C. Superior Court Judge Harriett R. Taylor has denied the government's motion to dismiss the case against Sarah B. Cyrus, who was sentenced to nine months' probation for possession of cocaine, saying that she does not understand how the dismissal would serve the purposes of justice.

Before Cyrus' trial in July, her attorney, Veronice A. Holt, had questioned many of the same 4th District police practices that are now being investigated by a federal grand jury.

In court papers filed in February, Cyrus said that her apartment on Fairmont Street NW had been searched four times between Oct. 31, 1986, and Jan. 15, 1987, and that in each instance the affidavits on which the search warrants were based were strikingly similar and consistently vague.

For example, the warrants stated only that an informer had purchased drugs from a "black female" in the apartment, and gave no identifying details about the woman and no specifics about the search.

On Jan. 15, the date of the search that resulted in the charge on which Cyrus was convicted, 4th District police also searched another apartment at 1401 Fairmont St. NW, and evidence from the search of Cyrus' apartment, 504, was mixed up with evidence from Apartment 404, according to court records.

On Oct. 31, Cyrus' apartment was one of four in her building that were searched after warrants were obtained based on virtually identical affidavits.

"The defendant's theory of the case will be that the assertion of recovery of drugs from #504 on Jan. 15, 1987, is a deliberately false allegation pursuant to a pattern of police harassment of the defendant," Holt said in court papers.

Several of the cases being dismissed involved searches that were made outside the 4th District. Sources have said that the 4th District officers, unlike vice officers in other districts, often conducted raids beyond their assigned territory. The usual procedure for other districts is to refer such cases to officers in the appropriate police district.

Cyrus' apartment is in the 3rd District. The apartment in which 5.7 kilograms of marijuana were seized, leading to the conviction of Jerry L. Jones, is also in the 3rd District. The case against Jones, who had been sentenced to six months in work release and a year of probation, was dismissed last week.

The sole case in federal court, which has resulted in the dismissal of charges against three men and the release from custody of each, involved more than 30 pounds of marijuana. Staff writer Karlyn Barker contributed to this report.