As CBS News first reported last spring, FEMA has been under heavy fire for failing to acknowledge then adequately address health problems like respiratory illness associated with the toxic chemical formaldehyde found in travel trailers that became home for hundreds of thousands of survivors of Hurricane Katrina. More than 143,000 families have lived in the toxic trailers, and more than 40,000 still do.

Now, CBS News has learned, the public health fiasco reaches beyond FEMA - into the one of the nation's most respected agencies.

CBS News has learned that the Centers for Disease Control, the nation's top public health agency, suppressed repeated warnings from one of its top scientists, raising questions about whether the CDC bowed to pressure from FEMA to conceal the long-term health risks of formaldehyde in the trailers it distributed to hurricane victims - health risks like cancer and birth defects, CBS News chief investigative correspondent Armen Keteyian reports.

A string of internal documents obtained exclusively by CBS News reveal that Dr. Christopher De Rosa, director of the CDC's Division of Toxicology and Environmental Medicine, told his superiors "there is no safe level of exposure" to formaldehyde in trailers. That warning never made its way into any public report about the trailers.

In addition, Dr. De Rosa wrote in an email that two of his staff members had been directed by FEMA officials to not "address longer term health effects" of formaldehyde in this February 2007 report.

"To not do its due diligence on this issue borders on malfeasance," said Rep. Bernie Thompson, D-Miss., chairman of the House Homeland Security Committee.

In fact, it wasn't until October 2007 - after eight months and pressure from congressional investigators that the CDC revised its February report and finally issued warnings about cancer and other long-term health risks of formaldehyde.

"For them to punt on this issue does not speak well for them as an agency," Thompson said.

De Rosa refused an on-camera interview with CBS News. The CDC did not comment on the documents, but said it changed the report after it realized there was a problem.