Six D.C. police officers who were assigned to the 4th District's vice squad have been notified by the FBI that they were the subject of audio or visual surveillance late last summer, apparently while executing search warrants at two Northwest locations, sources said yesterday.

The FBI letters said the surveillance took place at 1450 Irving St. NW, Apt. 312, and 3519 14th St. NW, Room 401, according to the sources, who said federal authorities apparently planted quantities of drugs and money at the two locations.

All six officers are among the 16 former vice squad members who have been transferred to administrative duties pending completion of the FBI's investigation of alleged drug and money skimming by some squad members.

The FBI investigation was disclosed by The Washington Post on Aug. 28, and 10 days later the FBI removed more than a truckload of documents from the vice squad's offices at the 4th District, 6001 Georgia Ave. NW. The surveillance disclosed in the letters may have taken place during that 10-day period, sources said.

Two 4th District vice officers, James Whitaker and Shelton D. Roberts, were relieved of their police powers as a result of the investigation and are believed to be the main targets of the probe. No indictments have been returned in the case.

G. Allen Dale, Whitaker's lawyer, said yesterday that his client had not received a letter from the FBI. Roberts' attorney, Douglas H. Behr, said he did not know if Roberts had been notified.

Sources said last night that Whitaker had been sent a letter.

The so-called inventory letters are sent to any persons picked up by court-authorized electronic surveillance equipment and do not indicate whether the person is a target of the probe.

Gary Hankins, head of the labor committee for the Fraternal Order of Police, confirmed last night that "several" 4th District officers had received what he called "generic letters" of notification. He also strongly criticized federal authorities for leaving police officers "twisting in the wind" for the past five months.

Under law, persons intercepted by court-authorized surveillance must be notified within 90 days, unless there are court-approved extensions of that period. The U.S. attorney's office said last night that it would not comment on whether letters were sent or if an extension was obtained.

"Most of the officers in the police department were willing to give the U.S. attorney's office the benefit of the doubt, to accept that there may have been bad police officers," Hankins said. "But now they are questioning openly whether {U.S. Attorney Joseph E.} diGenova had anything to begin with."

A federal grand jury has been hearing testimony in the case over the past five months, but there is no indication that indictments are expected soon.