Washington —- The Federal Emergency Management Agency manipulated scientific research to play down the danger posed by formaldehyde in trailers issued to hurricane victims, according to an investigation by congressional Democrats released Monday.

FEMA "ignored, hid and manipulated government research on the potential impact of long-term exposure" on Katrina and Rita victims now living in FEMA trailers, Democrats on a House Science and Technology subcommittee wrote in a letter to Homeland Security Secretary Michael Chertoff.

In a separate letter, lawmakers said the federal health agency that provided guidance to FEMA was "complicit in giving FEMA precisely what they wanted."

Victims living in FEMA trailers have complained of health problems related to formaldehyde, but initial FEMA tests revealed the air quality in the trailers was safe if the trailers were properly ventilated. Formaldehyde, a common preservative found in building materials used in manufactured homes, has been classified as a carcinogen by the International Agency for Research on Cancer and as a probable carcinogen by the U.S. Environmental Protection Agency.

FEMA said the health agency's Feb. 1, 2007, advice didn't address long-term health effects.

The lawmakers are questioning the integrity of research done by the Agency for Toxic Substances and Disease Registry, which they said ignored research from one of its own experts, Christopher De Rosa.

"Any level of exposure to formaldehyde may pose a cancer risk, regardless of duration," De Rosa wrote in a February 27, 2007, letter to a FEMA attorney. "Failure to communicate this issue is possibly misleading and a threat to public health."

In its initial round of testing, FEMA took samples from unoccupied trailers that had been aired out for days and compared them with federal standards for short-term exposure, according to the lawmakers. FEMA officials instructed scientists at the health agency to leave out details about long-term exposure, lawmakers said.

FEMA currently is testing 500 of the 40,000 trailers, but the lawmakers said they have no confidence in the new testing.