DENVER: Providing the first scientific testimony linking Timothy McVeigh to bomb materials, FBI chemist Steven Burmeister told jurors that when McVeigh was arrested after the Oklahoma bombing, his clothing carried traces of PETN, an explosive used in bomb detonator cord. It was a dramatic ending to a day that the defense spent in attacking the credibility of the embattled FBI crime lab, including accusations that FBI forensic scientists contaminated key pieces of the Ryder truck used in the bombing. Several of the shards of the truck found near the explosion site are instrumental to the prosecution's case because the FBI has said it found ammonium nitrate crystals on it, the same chemical fertilizer that the bomb contained. Defense attorney Christopher Tritico hammered away at FBI agents who collected the evidence, pointing out that several photos had to be staged because the pieces had been moved from their original spots. FBI chemist Ron Kelly acknowledged that an FBI photographer failed to take a picture of one piece of the truck's cargo hold before it was collected, and improperly documented a photograph of another piece. Realizing that the crime lab represented a weak link in their case after a highly critical report recently alleged that lab workers bungled numerous tests or selectively reported their findings, prosecutors had dropped several crime lab experts as witnesses. Burmeister, whose work was praised in that report, was their strong card.