The federal Centers for Disease Control are trying to find as many as 20,000 people who lived near the Hanford Nuclear Reservation and may have suffered radioactive contamination in the 1940s and early '50s.

Nuclear reactors on the reservation in Washington state produced the plutonium for the atomic bomb that destroyed Nagasaki, Japan, in 1945."A lot of iodine 131 was released into the atmosphere at Hanford," said Dr. Dan Hoffman, assistant director of the CDC's Center for Environmental Health and Injury Control, which will do the study.

He said research in Utah and the Marshall Islands in the Pacific indicates the radioactive isotope of normal iodine can cause thyroid problems.

Congress authorized the study, which Hoffman said may take five years and cost up to $6 million, this year.

He said there is no proof yet that there have been long-range damaging effects from radioactive materials leaked from the reservation near Richland, Wash.

"It's all anecdotal so far," he said. "The most difficult task will be to find the people, to reassemble the populations and try and track them down."

He said the CDC will rely heavily on birth and school records to try to find people who lived downwind of the reservation.

Iodine 131 has a half-life of only eight days, meaning it decays quickly and does not linger in the environment or travel far, he said.

Last May, about 50 people who lived in the path of Hanford releases met in Spokane to discuss possible related health problems.