Prosecutors in the drug trial of D.C. Mayor Marion Barry presented a witness yesterday who said that she had seen D.C. police Detective James L. Stays, a senior member of Barry's security detail and the first defense witness, use cocaine.

That testimony came as part of a government attempt to attack key defense witnesses -- including not just Stays but also Clifton A. Roberson, a Barry staff member. Roberson had testified earlier that it was he, not Barry, who accepted a job application from prosecution witness Lydia Reid Pearson on a morning in 1988 when she said she delivered crack to the mayor.

Meanwhile, sources told The Washington Post last night that Stays has been removed from Barry's security detail pending an internal investigation. The sources said Stays had been stripped of police powers and assigned to an administrative position.

Stays, who joined the police department in October 1960, is a senior member of the detail. Stays denied in his testimony last week that he had used drugs, and associates said Stays has told them he has been wrongly accused. Stays declined to comment last night.

In their efforts to prove Roberson wrong yesterday, prosecutors produced a D.C. government secretary who testified that she had typed a form showing Roberson was "absent without leave" -- laying a stone on his father's grave -- on the morning in question.

The government's rebuttal case began shortly after the mayor's attorney, R. Kenneth Mundy, announced that the defense had rested. Mundy also made official what had been suspected for some time: that the mayor would not testify.

"I have advised him that no matter how we might restrict or limit his testimony on direct {examination}, that he would be subject to cross-examination entirely," Mundy told U.S. District Judge Thomas Penfield Jackson. "We have carefully considered it and we have discussed it numerous times, and it is his informed decision that he will not take the stand."

After court had recessed for the day, Barry told reporters that he did not regret not taking the stand.

"The burden is on the government to prove Marion Barry guilty of all 14 counts, and my lawyer felt they had not done that, and therefore, no need for me to do it."

The government witnesses' attack on Stays and Roberson prompted Mundy to ask Jackson for a chance to put on still more witnesses to counter the government's allegations. But at the end of the day, Jackson told the jurors he still hopes to complete the case and start deliberations early next week.

Earlier in the trial, Stays testified that in his 11 years on Barry's security detail he had never seen the mayor use illegal drugs. But his testimony was almost immediately called into doubt when Assistant U.S. Attorney Judith E. Retchin, outside the jury's presence, told the judge that prosecutors had evidence Stays had used cocaine himself.

But yesterday, Delois Ann Mise, a former civilian employee of the D.C. police department, testified that she and Stays used cocaine together at an August 1985 cookout in Kettering.

Mise testified that she had gone to the cookout with a female friend. Shortly after the two arrived, she said, they went around the corner of the house with Stays and another man whose name she did not know, and all four of them snorted cocaine.

Mise also testified that she was present at the Omni Shoreham Hotel in Northwest Washington in the spring of 1983, where, she said, D.C. employees were having a private party. She said she caught a glimpse of Stays entering a room where Barry was standing near a table that held a plastic bag of what she took to be powdered cocaine.

Barry has said that Stays offered to take a drug test when he was interviewed by FBI agents and D.C. police detectives, but authorities have said Stays refused the test. Barry said that during Stays's questioning by the investigators, he was searched for a hidden listening device.

Before Mise took the stand yesterday, prosecutors called another witness in an effort to contradict Roberson's story: Darcell Walker, a secretary in Roberson's office.

Obviously nervous, Walker said she had been subpoenaed to testify only this morning. She testified that she typed up an "absent without leave" form that showed that Roberson was out laying a stone for his father's grave on the morning that, according to his testimony, he accepted a job application from prosecution witness Lydia Pearson.

Pearson, a self-described former cocaine addict, earlier testified that she delivered three $30 bags of crack cocaine on the morning of Sept. 7, 1988, to Barry at his office at the Frank D. Reeves Municipal Center at 14th and U streets NW.

The form that Walker testified that she typed contained a typed line stating that Roberson was AWOL from 4:30 to 6:30 p.m. That time was later crossed out and a handwritten notation was added that reads "8:30 to 10:30 a.m."

It also contained a handwritten note that said, "Laying a gravestone for my father's grave. Turned in leave slip which was rejected. Signed under protest. CR."

Walker testified that she remembered the handwritten note was on the copy she received back from Roberson, and that Roberson was upset when he returned it.

On cross-examination, Mundy produced a time sheet signed by Walker showing that Roberson was docked for two hours of pay that day, but that the two-hour period he was absent was 4:30 to 6:30 p.m.

That did not end the matter. As Mundy was cross-examining Walker, prosecutors were issuing a subpoena for yet another city employee in another agency for another copy of the AWOL form.

The resulting witness was Eugene Bowers, an employee in the D.C. Office of Personnel, who said he had received his subpoena only an hour before appearing in court. Bowers said that forms in the city's personnel folder showed that Roberson was indeed absent from 8:30 to 10:30 a.m. that day.

Mundy indicated that he expected to recall Roberson to the stand on Monday to explain the discrepancy. Jackson called the new evidence "a rather startling revelation . . . . I think it reflects badly on Mr. Roberson."

As the trial ended its sixth week of testimony, both sides said the presentation of evidence was nearly complete. The prosecution is expected to complete its case Monday, with closing arguments Wednesday.

Earlier in the day, Mundy took his last chance to question FBI agent Ronald Stern, who oversaw the Vista Hotel sting last Jan. 18 in which Barry was arrested, as an opportunity to walk Stern through a minute-by-minute account of why agents failed to call off the operation.

In his questioning of Stern on Thursday, Mundy had elicited the agent's acknowledgement that it was apparent Barry was more intersted in sex than in drugs. Mundy also had sought to suggest to the jury -- indirectly, through his questions -- that the agents should have arrested the mayor before he used the crack because they had evidence of simple drug possession.

Yesterday morning, under friendly questioning from Retchin, Stern was given a chance to say that agents were waiting to see what Barry might do with the crack cocaine -- whether he would flush it down the toilet, chastise Hazel Diane "Rasheeda" Moore for using drugs, walk away or use the drugs himself.

"Did you wait to arrest Mr. Barry to give him an opportunity not to use the drugs?" Retchin asked.

"Yes," said Stern. "Yes."

But Mundy seized on that rationale when he got his second chance to question Stern.

"Sir, honestly and candidly, did it seem likely to you that he would give $30 and then flush the drugs down the toilet?" Mundy asked.

"We weren't sure what he would do with the drugs," Stern replied, adding, "Anything could happen at that stage, Mr. Mundy."