More than a year ago, officials inside the Centers for Disease Control and Prevention discussed the potential long-term cancer risk posed by trailers housing thousands of Hurricane Katrina survivors, records show.

But the Atlanta-based agency delayed an investigation of the threat of long-term exposure to formaldehyde until recently because CDC needed an official request before it could act, said CDC media relations director Glen Nowak.

"One of the challenges for CDC all the time is we come in at the behest of someone else. ... Someone has to invite us," Nowak said Thursday, following the agency's announcement that residents need to move out of the trailers as soon as possible.

The CDC's view of its limited ability to take initiative to protect the public drew surprise and criticism from some public health experts and a member of Congress.

"It's just an indefensible position," said Michael Greenberger, director of the Center for Health and Homeland Security at the University of Maryland School of Law.

U.S. Rep. Bart Gordon, chairman of the House Committee on Science and Technology, said Thursday that CDC's findings that the trailers were unsafe for people to live in should have been made more than a year ago.

His committee is investigating allegations that CDC and the Federal Emergency Management Agency, which purchased the trailers, conspired to delay or avoid investigating the cancer threat.

"These agencies knew since the spring of 2006 that Hurricane Katrina and Rita victims living in those trailers were getting sick," said Gordon (D-Tenn.).

In 2006, FEMA requested that CDC provide advice about formaldehyde risks — but only asked for CDC's assessment of risks posed by short-term exposures of two weeks or less.

Records obtained by congressional investigators show that CDC toxicologist Christopher De Rosa alerted FEMA officials in 2006 that any assessment of formaldehyde would need to include the long-term risks of cancer. But CDC officials, bypassing De Rosa, in February 2007 gave FEMA the report with just short-term risks addressed.

Only after De Rosa repeatedly pressed the issue last year did CDC and FEMA in December begin testing hundreds of trailers that led to Thursday's health warning, records show. Gordon's committee is investigating whether CDC officials have been retaliating against De Rosa since last fall, a charge the CDC denies.

Nowak said CDC's job is to follow the directions of those, like FEMA or state health departments, who request the agency's help. He said CDC doesn't have the authority to act on its own to investigate a health threat.

Those authorities lie primarily with state health departments, said James Hodge, executive director of the Center for Law and the Public's Health at Johns Hopkins Bloomberg School of Public Health. CDC works at the request of local health departments or other federal agencies, he said.

Jeffrey Levi, executive director of the Trust for America's Health, a watchdog group based in Washington, D.C., disagreed that CDC's hands are tied.

"The whole point of having the Centers for Disease Control is to identify risks to the public health, and the agency's mandate is not limited to a narrow question that is asked," Levi said.