The U.S. attorney's office has been told that some vice squad officers in the city's 4th Police District routinely lied under oath to judges to obtain search warrants, according to sources.

In some cases, prosecutors have been told that the confidential informants cited in the warrants didn't exist; in other cases, they were told, the informants may have been real but the alleged drug purchases that they were said to have made didn't happen. The bogus information was allegedly used to justify obtaining a warrant to search a home.

Sources said the alleged pattern of officers using bogus affidavits raises major constitutional questions about the vice operations in the 4th Police District and was one of the primary reasons behind U.S. Attorney Joseph E. diGenova's controversial decision this week to drop 300 to 400 drug cases.

In some instances, the officers obtained several warrants for the same location over several months or different apartments in a single building within several days, swearing in each instance that they have witnessed drug purchases by unnamed informers, but providing almost identical descriptions.

In another police-related development, two employes of the D.C. police department's drug screening program who warned city and federal officials that test results in the program had been tampered with were relieved of their duties Wednesday, but will be reinstated Monday. {Story on Page B1.}

In the case of the warrants, sources said the procedure may explain how the 4th District was able to obtain more than 300 search warrants in the first eight months of this year, when other districts with heavy drug trafficking areas obtained considerably fewer. For example, the 7th Police District, which covers much of Southeast Washington, obtained 120, the second-highest number in the same period.

A spokesman for the D.C. police said the department would have no comment because "that's under investigation." A spokesman for the U.S. attorney's office declined to comment.

This is the first indication that the investigation of alleged skimming of drugs and money by some 4th District police officers has broadened to include possible perjury and obstruction of justice charges. Sources said investigators are trying to determine if the informers cited in many of the affidavits actually exist.

A source in the police department administration confirmed yesterday that all 16 members of the 4th District vice unit will be detailed elsewhere in the department.

To obtain a warrant in either D.C. Superior Court or U.S. District Court, a D.C. police officer must first ask an assistant U.S. attorney to approve the request and must provide a sworn affidavit listing the proof that a crime has been committed or is being committed. The officer must swear to the truthfulness of the affidavit before a judge or magistrate, and the warrant must be executed within a certain period -- usually 10 days -- after it is issued.

The Fourth Amendment guarantees that a person's home may be searched only when an officer can show in a sworn warrant that there is "probable cause" to believe a crime is being committed. The Supreme Court has ruled repeatedly that evidence gathered in violation of the Fourth Amendment protections must generally be thrown out.

DiGenova's decision to drop the cases, roughly 10 percent of all drug cases pending in D.C. Superior Court, has brought sharp criticism from Mayor Marion Barry and some police union officials who claim that the move is unjustified and will put dangerous drug dealers back on the streets.

A federal grand jury yesterday began hearing testimony in the police corruption probe and sources said indictments could come within several weeks.

A review of court documents for several local and federal cases in the past year shows that defense attorneys have repeatedly questioned the truthfulness of affidavits submitted by 4th District vice officers, claiming that many of the affidavits are almost identical, that they provide few details and that in many instances the events described could not have taken place.

In March, a local defense attorney circulated a letter asking lawyers in the city's Public Defender Service to help document the "distressingly high number of similar factual allegations" and specifically questioned the affidavits submited by three 4th District officers. Two of the men, Officers Shelton D. Roberts and James Whitaker Jr., have been identified as subjects of the current probe and have been relieved of their police powers.

A month earlier, defense attorney Veronice A. Holt cited nearly identical affidavits filed by 4th District vice officers -- including Whitaker and Roberts -- to support her claim that officers in the unit were making "material false statements . . . intentionally, or with a reckless disregard for the truth."

Several attorneys said this week that requirements necessary to obtain search warrants mean that many affidavits will be similar, but that the only differences in affidavits submitted by 4th District vice officers were often just the names and addresses.

In addition, the 4th District officers often described incidents in which what appears to be the same informant made separate drug purchases within the same apartment building within the same short period of time, and with the same officer observing. Defense attorneys familiar with vice operations say such a scenario is highly improbable.

For example, 4th District officers searched four apartments of 1401 Fairmont St. NW on Oct. 31, and each of the affidavits states that an informer purchased a white powder from a black female within the same three-day period. Three of the apartments, 503, 504, and 507, are on the same floor of the building.

The affidavits differ only in the addresses and in describing the informer as either "confidential" or "reliable." Each of the affidavits was signed by the same officer, Ronald C. James, who swore that he had witnessed each of the purchases.

In papers filed in D.C. Superior Court seeking a dismissal of the case, defense attorney Thomas Heslip said the procedure described in the warrants "seems highly unlikely, since it would result in a high probability that the source would be identified."

" . . . Such a procedure would represent an incredibly reckless disregard for the safety of a 'source,' " Heslip said.