In a "Dear Bill' letter hand-delivered over the weekend, five Alexandria staff prosecutors have threatened to resign if the city's embattled commonwealth's attorney, William L. Cowhig, returns to his official duties.

The letter, which also was circulated to the city's Circuit Court judges and City Manager Douglas Harman, drew no immediate comment from Cowhig. Cowhig stepped aside temporarily last summer pending the outcome of three felony charges against him.

The letter was the latest in a series of developments that have buffeted the 53-year-old Cowhig since his indictment last Aug. 3 for alleged irregularities in the city's lucrative bingo operations.

Cowhig was acquitted of a felony bribery charge in Alexandria circuit Court in December, and won acquittal again last month on a charge that he had violated Virginia's gambling statutes in connection with a bingo operation on Duke Street in Alexandria.

A member of Cowhig's defense team called for Cowhig's resignation shortly after his December trial, a move that won support from Roger Amole, president of the city's 300 member bar association. But Cowhig refused to step down.

Cowhig faces a third trial beginning March 5 on a separate gambling charge.

Cowhig's long-time administrative aide, Mary Ann (Sam) Pastorek, was placed on a 30-day administrative leave yesterday, but city officials declined to give any reasons for the move. Pastorek, who has continued to work in her office during Cowhig's absence, was seen leaving City Hall at noon with Cowhig at her side. She refused to speak to a reporter as she fought back tears.

Asked if he had any comment, Cowsaid, "Not today."

In their letter, the five attorneys said:

"Dear Bill: We want you to know that if you resume direction of the Office of the Commonwealth's Attorney, we will resign immediately from our respective positions in the office."

The letter was signed by Acting Commonwealth's Attorney John E. Kloch, Cowhig's former deputy, and assistant prosecutors Thomas Rawles Jones Jr., Richard S. Mendelson, David W. O'Brien and Frank Andrew Carroll III.

Two assistant prosecutors, A. John Scariot and Connie Frogale, did not sign the letter.

Scariot said yesterday he was not informed of the resignation threat in advance. "They knew I wouldn't sign it. My conscience wouldn't let me. He's been acquitted of two charges and is still facing a third trial," he said.

Scariot said Cowhig had called him Sunday night after receiving the letter. "Bill sounded upset, but gave me every indication he would stick it out."

Kloch's letter was the first public statement from the commonwealth's attorney's office on Cowhig's controversial role in the city's bingo scandal.

Kloch declined further comment yesterday and city officials also remained tight-lipped.

Sources close to the commonwealth's attorney's office said, however, that the prosecutors were troubled by Cowhig's failure to take the stand in his own defense during the bribery and gambling trials.

There also was speculation that Cowhig, even if acquitted of the third charge, would have a difficult, if not impossible task in working with the police department, city manager and City Council.

It was learned yesterday that defense lawyers for Cowhig have asked Circuit Court Judge Percy Thornton, who presided over Cowhig's first two trials, to "expunge" Cowhig's arrest records and court documents in light of Cowhig's two acquittals.

In opposing the motion, Special Prosecutor Edward J. White, in court papers filed last week, said "The petitioner's failure to testify and failure to rebut evidence of questionable activity most certainly should be taken into consideration."

Thornton has not ruled on the motion.

City Council member Donald C. Casey said yesterday that he "admired" Kloch and the assistant attorneys for "taking a stand." Casey added: "It's up to Mr. Cowhig now. I hope he takes the right course of action. The handwriting's on the wall."

Copies of the letter went to Judges Wiley Wright, Donald Kent and Albert Grenadier, who was elected to a judgeship last week; City Manager Harman, Alexandria Police Chief Charles T. Strobel, bar president Amole and Alexandria Sheriff Michael Norris. All declined to comment yesterday.