Government researchers are investigating whether as many as 20,000 people who lived near the Hanford nuclear reservation in Washington state in the 1940s and early 1950s were contaminated by radioactive spills. The federal Centers for Disease Control in Atlanta said the leakage over about nine years of several radioactive elements, especially Iodine 131, apparently is the most massive radioactive leak of any facility in the country. The reservation opened in 1944. Dr. Dan Hoffman, assistant director of the CDC’s Center for Environmental Health and Injury Control, said research in Utah and the Marshall Islands in the Pacific indicated Iodine 131 can cause thyroid problems. Records of the Hanford leak and the size of the dose were not released until a Spokane, Wash., environmental group obtained them in 1986 under the Freedom of Information Act.