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. Full Story