U.S. Attorney Joseph E. diGenova announced yesterday that 300 to 400 drug cases will be dismissed in an unprecedented response to allegations that some narcotics officers in the city's 4th Police District may have stolen drugs and money during raids.

In addition, law enforcement sources said yesterday that D.C. Police Chief Maurice T. Turner Jr. is strongly considering replacing the entire 12-member vice unit in the 4th District as a result of the ongoing probe.

DiGenova told Turner in an afternoon meeting that prosecutors will be instructed to begin dismissing today all pending drug cases based on evidence gathered by the 4th District vice squad, the target of an FBI investigation into the actions of four or five officers and one of the most active police stations in the city. One case alone involves the seizure of $250,000 worth of drugs.

Law enforcement officials said the action appeared to be the first time in the city's history that criminal charges involving an entire police squad will be dismissed because of allegations of wrongdoing.

"None of us likes having to dismiss drug cases, but when you have a situation like this that involves the integrity of the judicial process, all of us in law enforcement have higher duties to the system that must be fulfilled," diGenova said.

Turner declined to comment through a spokesman, but diGenova said in an interview that "he and I both agreed that it needed to be done and that he would take additional steps on his own."

Sources said yesterday that those steps could include the replacement of the 12 officers on the vice squad. No officers have been charged in the investigation, although the police powers of two narcotics officers have been revoked.

The FBI investigation centers on allegations that four or five 4th District narcotics officers may have profited from drug raids and that some received payoffs in exchange for information about drug sweeps. Law enforcement officials suspect that others on the squad may have been aware of the activity but were not actively involved.

Reaction to diGenova's announcement was swift among city officials who have targeted illicit drug use as one of the city's most vexing problems.

Mayor Marion Barry said through his spokesman John C. White that "it would be regrettable that people who should be prosecuted escape prosecution."

D.C. Council Chairman David A. Clarke said: "If this had to be done, I would want it done on a thorough examination on a case-to-case basis rather than on the basis of a 'computer run.' Secondly, while I understand that a prosecutor may on occasion have to let one evil go to get at a bigger evil, it must be a damned big evil to drop 400 narcotics cases."

DiGenova, who announced his decision after nearly daily meetings among office supervisors, said many of the dismissed cases will be reviewed after the FBI investigation is completed to see if "any of them can be saved or salvaged."

DiGenova added, however, that "we're not holding up any promises." He defended his decision by saying that the dismissals should send a message to the community that "when cases are made, they are made fairly and according to the law."

Sources said a decision was made to dismiss all drug arrests by 4th District narcotics officers, even though most of those officers have not been implicated, because defense lawyers could challenge the cases by claiming that the other officers may have known about the alleged skimming.

It was unclear how much narcotics and seized money might be involved in the dismissals, but one case dismissed yesterday in U.S. District Court involved the seizure of drugs worth more than $250,000. Anthony B. Fultz, the defendant, had charged in court papers that more than $1,000 had been stolen from him during the search of his home by Officer Shelton D. Roberts. Other sources said that in some cases, as much as $5,000 was allegedly stolen.

Roberts, along with his partner Officer James Whitaker Jr., has been identified as a prime subject of the FBI probe; both men have been relieved of their police powers.

The 4th District, with headquarters at 6001 Georgia Ave. NW, covers sections of upper Northwest and Northeast Washington. Narcotics officers there have executed about one-third of all drug search warrants issued in D.C. Superior Court and U.S. District Court this year.

DiGenova said no decision had been made about what to do with narcotics cases that have resulted in guilty pleas or convictions. Most of the cases that will be dropped involve arrests made this year.