NEW ORLEANS (AP) — BP asked a federal judge Thursday to sanction its cement contractor on the ill-fated Deepwater Horizon drilling project, accusing the company of withholding critical evidence that could have been used at the ongoing trial over the nation's worst offshore oil spill. In a late-night court filing, BP PLC lawyers cited Halliburton's alleged destruction of cement samples as grounds for U.S. District Judge Carl Barbier to rule that the contractor's cement design on the drilling project was unstable before the April 2010 blowout of BP's Macondo well. A Halliburton lawyer disclosed that the company discovered cement samples last Wednesday at a Lafayette laboratory that weren't turned over to the Justice Department for testing after the spill. BP says its inability to test the missing samples has harmed its ability to defend itself at the trial, which Barbier is presiding over without a jury.