Ten days before Timothy J. McVeigh was executed for the Oklahoma City bombing, lawyers for F.B.I. laboratory employees sent an urgent letter to the attention of Attorney General John Ashcroft, saying a crucial prosecution witness might have given false testimony about the security of forensic evidence.

The accusations, which involved Steven Burmeister, now the F.B.I. laboratory's chief of scientific analysis, were never turned over to Mr. McVeigh, though they surfaced as a judge was weighing whether to delay his execution because the government withheld evidence.

The letter, however, was recently turned over to Terry L. Nichols, who faces a murder trial in the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, which killed 168 people.

''Material evidence presented by the government in the OKBOMB prosecution through the testimony of Mr. Burmeister appears to be false, misleading and potentially fabricated,'' said the June 1, 2001, letter to Mr. Ashcroft, which was obtained by The Associated Press.

The letter said Mr. Burmeister's testimony in an unrelated civil case contradicted his earlier testimony about Mr. McVeigh.

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At the McVeigh trial, Mr. Burmeister testified that his work area was restricted and that laboratory employees who entered had to wear protective clothing.

The law firm letter to Mr. Ashcroft said Mr. Burmeister had testified in the civil case that cleaning crews and a fellow chemist had unrestricted access to his work area, without protective clothing.

The letter was sent to Mr. Ashcroft by fax and by courier with the notation ''urgent matter for the immediate attention of the attorney general.''

Justice officials said today that the letter was routed to Mr. Ashcroft's clerical office in the Maryland suburbs of Washington, where it sat for nearly two months, and was then forwarded to the Federal Bureau of Investigation, well after the day Mr. McVeigh was executed, June 11, 2001.

Neither Mr. Ashcroft nor other top officials in the Justice Department who handled the McVeigh case saw the letter, said a department spokeswoman, Barbara Comstock. The letter was never reviewed to determine if it should be handed over to Mr. McVeigh's lawyers, officials said.

Prosecutors are obligated by law to disclose any potentially exculpatory evidence to the defense.

Mr. McVeigh's lawyers expressed dismay at the handling of the letter. At the time it was sent, a judge had delayed Mr. McVeigh's execution by one month because of other evidence that the F.B.I. had not turned over.

''It is truly shocking and just the latest revelation of government conduct that bankrupts the prosecution, investigation and verdict,'' said Stephen Jones, Mr. McVeigh's lead trial lawyer.

After the trial, Mr. McVeigh admitted to the bombing in interviews.

Ms. Comstock said the Justice Department did not believe the accusations about testimony would have affected the trial's outcome.

''Court after court has found that the evidence of guilt against McVeigh was overwhelming,'' she said.

F.B.I. officials stood by Mr. Burmeister.

''It didn't happen,'' the director of the F.B.I. laboratory, Dwight Adams, said when asked about the accusations of contradictory testimony. ''Steve Burmeister is one of the F.B.I.'s finest experts. He is meticulous and honest.''

The accusations surfaced in mid-May 2001, when Mr. Burmeister was being questioned by lawyers for F.B.I. laboratory employees who had sued the agency. One of the employees had been dismissed.

The law firm that sent the letter represents several F.B.I. laboratory employees, including Frederic Whitehurst, the F.B.I. chemist who trained Mr. Burmeister and later made accusations that led to an overhaul of the laboratory.

Mr. Burmeister rose to prominence in the Oklahoma City case after he discovered ammonium nitrate crystals embedded in a piece of the Ryder truck that Mr. McVeigh used to deliver the bomb.

Mr. Burmeister's discovery was crucial to the government's case that Mr. McVeigh and Mr. Nichols had used a giant fertilizer bomb, with ammonium nitrate as a main ingredient.

Mr. McVeigh's defense lawyers attacked the evidence, suggesting that the ammonium nitrate, which dissolves in moisture, could not have survived the rain that fell in Oklahoma City shortly after the bombing and that it might have come from contamination in the laboratory.

Some forensic evidence was kept out of the McVeigh trial because of possible contamination, but Mr. Burmeister's testimony was permitted. He told the court that the evidence he found could not have been contaminated because he kept his examination area locked. He said that only F.B.I. personnel wearing sterile laboratory coats and other protective gear had access to the area.

Outsiders ''were restricted basically from coming into my work area, into my room,'' Mr. Burmeister testified.

If laboratory employees are ''coming into my area where I'm going to be handling evidence, they're required to wear protective clothing,'' he added.

However, the law firm's letter to Mr. Ashcroft cited sworn testimony from Mr. Burmeister that appeared to contradict his statements in the McVeigh trial.

''Mr. Burmeister testified that while this chemist shared an office with him, no extra precautions were taken to prevent contamination,'' the letter said.

The letter quoted Mr. Burmeister's description of the access cleaning crews had to his work area:

''There was a service staff that would come in and buff and wax the floors,'' he testified, adding that sometimes crews came in while he was working in his suite.

''I've known them to clean windows in offices, and I've seen them stand on the heating elements by the windows,'' Mr. Burmeister was quoted as saying.

Ammonia is one of the ingredients in common window cleaning solution.

Mr. Adams, the F.B.I. laboratory director, said that the lawyers had taken Mr. Burmeister's deposition testimony out of context and that the fact that cleaning crews or laboratory employees in nonsterile clothing entered his work area after hours could not account for how the ammonium nitrate crystal became embedded in the truck part.

''You can make what you want to about who has access to the room,'' Mr. Adams said, ''but the key fact is that crystal was embedded with great force, and that could only come from an explosion.''

The lawyers' letter said Mr. Burmeister contradicted testimony on another matter when he testified that the chemical PETN, which was found on Mr. McVeigh's clothing, is ''not used for drug purposes anymore.''

In his deposition, Mr. Burmeister admitted that PETN, a chemical found in detonation cords, was also ''stil used for some heart medications,'' the letter to Mr. Ashcroft said.

The letter questioned why prosecutors had never corrected the trial record.

Mr. Adams said that Mr. Burmeister had qualified his answer at the McVeigh trial by saying it was to the best of his knowledge and that other witnesses explained to the jury about the multiple uses of PETN.

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