WASHINGTON, D.C. --- Invoking executive privilege, the U.S. Environmental Protection Agency has refused to provide lawmakers with a thorough explanation of why it rejected California's greenhouse gas regulations, the San Francisco Chronicle reported. The EPA told U.S. Senator Barbara Boxer (D-Calif.) that many of the documents she had requested contained internal deliberations or attorney-client communications that would not be shared with Congress. "EPA is concerned about the chilling effect that would occur if agency employees believed their frank and honest opinions and analysis expressed as part of assessing California's waiver request were to be disclosed in a broad setting," EPA Associate Administrator Christopher Bliley wrote. The EPA gave the Senate Environment and Public Works Committee, which Boxer chairs, a box of papers with large portions of the relevant documents deleted, Boxer said. The Chronicle reported that the documents omitted key details, including a presentation that, according to Senate aides, predicted the EPA would lose a lawsuit if it was challenged in court for denying California's waiver. The EPA's refusal to provide a full explanation is the latest development in a congressional investigation into why the EPA denied California permission to impose what would have been the country's toughest greenhouse gas standards on cars, trucks and sport utility vehicles, the Chronicle reported. In denying the waiver in December, EPA Administrator Stephen Johnson told California Governor Arnold Schwarzenegger that the federal government is implementing a national fuel efficiency standard. Johnson's decision prompted congressional investigations and a legal challenge by California and 15 other states.