DENVER: Jury selection continues under tight security in the Oklahoma City bombing case as the government braces for possible trouble Saturday, the anniversary of both Waco and the Murrah building bombing. "There's a U.S. Marshal on every corner, for 2-3 blocks in every direction," reports TIME's Pat Cole from the federal courthouse in Denver. "Air space over the court house is off limits, and if you stop to talk on the corner outside the courthouse, police tell you to keep moving." Jury selection is expected to conclude Tuesday, with opening statements to begin on Thursday. Despite the recent flap over evidence treatment at the FBI's Crime lab, Cole says the prosecution's strategy should remain essentially unchanged. Once significant alteration: Prosecutors have dropped as a witness David Williams, the head of the FBI explosives unit who concluded the blast was caused by an ammonium nitrate-fuel oil bomb. A Justice Department report earlier this week said Williams made scientifically unsound conclusions in favor of the prosecution. Because prosecutors had anticipated problems with the crime lab evidence, they made arrangements late last year to call British explosive expert Linda Jones to confirm the FBI's report. "Prosecutors are still going to call more than 300 witnesses," says Cole. "The chemical evidence is some of their best, and that's what the defense will come after, so they need help from independent sources. The key for prosecutors will be to find some pieces of evidence that are still above contamination."