I wrote about how the EPA denied a waiver to the state of California so that they could enforce stricter emission controls, so the state sued. Now Sen. Barbara Boxer wants the EPA to explain why they've denied the waiver for the first time in 32 years. And just like every other department in the federal government, they've effectively decided that the legislative branch has no right to oversight and claimed executive privilege in refusing to release the information to Boxer.

Invoking executive privilege, the U.S. Environmental Protection Agency refused to provide lawmakers Friday with a full explanation of why it rejected California's greenhouse gas regulations.

The EPA informed Sen. 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.

More than a week after a deadline set by Boxer, the agency gave the Senate Environment and Public Works Committee, which she chairs, a box of papers with large portions of the relevant documents deleted, Boxer said. The documents omitted key details, including a presentation that, according to Senate aides, predicted EPA would lose a lawsuit if it was taken to court for denying California's waiver.

The refusal to provide a full explanation is the latest twist 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.