WASHINGTON - Amid intensifying scrutiny of its failure to act on climate change, the US environmental protection agency (EPA) has ordered employees not to talk to internal auditors, Congress or the media, according to a leaked email released yesterday by green campaigners.

The EPA has refused repeated requests from Congress to explain its December denial of California's request to regulate greenhouse gas emissions - a move that overruled the agency's own career scientists.

Three Democratic senators have scheduled a press conference today to discuss the controversy.

On June 16, after an email from the campaign group Public Employees for Environmental Responsibility (Peer), the EPA told its enforcement officials not to answer questions on the issue - even those from the agency's in-house auditors.

"If you are contacted directly by the [auditors'] office or [congressional investigators] requesting information of any kind ... please do not respond to questions or make any statements," the email said.

Enforcement officials were told to refer all questions to specific EPA representatives.

The issue is whether White House officials, including aides to vice-president Dick Cheney, improperly influenced the process by pressuring the EPA to reject California's bid to regulate emissions.

Peer, a non-profit group founded to fight political influence on government scientists, called the email proof of a "bunker mentality" at the agency.

"Inside the current EPA, candour has become the cardinal sin," Peer's executive director, Jeff Ruch, said.

"The clear intention behind this move is to chill the cubicles by suppressing any uncontrolled release of information."

Peer questioned whether the email could be a illegal obstruction of the EPA inspector general (IG), which conducts independent audits of the agency. IG auditors are given broad freedom under US law to examine internal policies at government agencies.

The EPA spokeswoman Roxanne Smith said the email was partly a response to a 2007 report by agency auditors on how to streamline communications. That report made no specific suggestions about how the EPA could make its process more efficient, however.

"There is nothing in the procedure that restricts conversation" between EPA staff and investigators, Smith said via email. "The procedure simply ensures timely responses and assists in tracking and record-keeping obligations."

One of the senators set to discuss the issue today, the environment committee chairman Barbara Boxer, accused the EPA chief, Stephen Johnson, of kowtowing to industry opponents of carbon regulations.

"Stephen Johnson is turning the EPA into a secretive, dangerous ally of polluters instead of a leader in the effort to protect the health and safety of the American people," Boxer, from California, said.

© 2008 The Guardian