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LOS ANGELES - US federal officials are investigating whether documentary maker Michael Moore, whose 2004 film Fahrenheit 9/11 attacked the Bush administration, broke laws when he went to Cuba for a new movie about US health care, according to a letter posted on Moore's website.

In the letter posted today, the US Treasury Department warns Moore that US citizens are barred from travelling to the communist country without government approval, and officials are concerned he went there in March 2007 without proper authorization. Violating the law "may result in civil and/or criminal penalties," the letter said.

Stamped May 2, the letter asks for details about travel dates, people on the trip and reasons Moore might qualify for a journalist license to go to Cuba.

A spokesman for Moore declined to comment on what the filmmaker was doing in the foreign country but said recent news reports about Moore taking victims of the September 11 attacks on New York's World Trade Centre to get care in Cuba were inaccurate.

In a response on Moore's website, Meghan O'Hara, the producer of the new film called SiCKO, said investigation was politically motivated. Moore was not available for comment.

A Treasury Department spokeswoman declined to comment on any specific enforcement action against Moore.

The agency "issues hundreds of letters each year asking for additional information when possible sanctions violations have occurred," said Treasury spokeswoman Molly Millerwise. US restrictions on Americans travelling to Cuba are an extension of the 1962 US embargo against the communist country.

An Oscar winner for the 2002 anti-gun documentary Bowling for Columbine, Moore is well-known for works that attack corporations and politicians.

Moore first gained fame with 1989's Roger & Me, which looked at automaker General Motors Corp's layoffs of factory workers in Moore's hometown of Flint, Michigan.

Fahrenheit 9/11 proved his biggest hit, raking in US$222 million at worldwide box offices in 2004 amid a storm of election year publicity and controversy. It attacked Bush's actions following the September 11 attacks, as well as the US president's handling of the Iraq invasion.

SiCKO, which premieres at this month's Cannes film festival in France, takes aim at the US pharmaceutical and health-care industry. It opens widely in theatres on June 29.

Ken Johnson, senior vice president of the Pharmaceutical Research and Manufacturers of America, said in a statement today that any review of America's health care system should be "balanced, thoughtful and well-researched."

"You won't get that from Michael Moore," Johnson said.