The federal agency responsible for regulating U.S. offshore oil drilling repeatedly ignored warnings from government scientists about environmental risks in its push to approve energy exploration activities quickly, according to numerous documents and interviews. Minerals Management Service officials, who can receive cash bonuses in the thousands of dollars based in large part on meeting federal deadlines for leasing offshore oil and gas exploration, frequently changed documents and bypassed legal requirements aimed at protecting the marine environment, the documents show. This has dramatically weakened the scientific checks on offshore drilling that were established under landmark laws such as the Marine Mammal Protection Act and the National Environmental Policy Act. Interviews and documents show numerous examples in which senior officials discounted scientific data and advice -- even from scientists elsewhere in the federal government -- that would have impeded oil and gas companies drilling offshore.