Moore, who made the hit documentary "Fahrenheit 9/11" assailing President Bush's handling of Sept. 11, said in the letter the White House may have opened the investigation for political reasons.

U.S. citizens are generally barred from travelling to the communist country unless approved by the government under a broad trade embargo imposed against Cuba since 1962.

The Treasury Department's Office of Foreign Assets Control (OFAC) notified Moore in a letter on May 2 that it was conducting a civil investigation for possible violations of the trade embargo restricting travel to Cuba.

"This office has no record that a specific license was issued authorizing you to engage in travel-related transactions involving Cuba," Dale Thompson, OFAC chief of general investigations and field operations, wrote in a letter to Moore.

The letter noted that Moore applied Oct. 12, 2006, for permission to go to Cuba "but no determination had been made by OFAC." Moreover, Moore sought permission to travel there under a provision for full-time journalists, the letter added.

"For five and a half years, the Bush administration has ignored and neglected the heroes of the 9/11 community," Moore said in the letter. "These heroic first responders have been left to fend for themselves, without coverage and without care.

"I understand why the Bush administration is coming after me - I have tried to help the very people they refuse to help, but until George W. Bush outlaws helping your fellow man, I have broken no laws and I have nothing to hide."

Moore was in Cuba in March to film a segment for his new documentary "SiCKO," which seeks to "expose the health care industry's greed and control over America's political processes."

Moore's spokesman Chris Lehane declined comment on the exact events that took place in Cuba during the filmmaker's visit. But he did say Moore brought with him some "9/11 heroes who had serious health problems."