By SUSAN MONTOYA BRYANALBUQUERQUE, N.M. (AP) a¿¿ Thirteen employees who were exposed to radiation during a leak at the nation's only underground nuclear waste dump aren't likely to experience any health effects, federal officials said Wednesday. The U.S. Department of Energy confirmed last week that 13 workers were exposed when radiation leaked from the Waste Isolation Pilot Plant. Testing on follow-up samples taken from the employees came back negative for plutonium and americium, the two radioactive isotopes that were detected in preliminary tests. The latest samples have been sent to the Centers for Disease Control and Prevention to be verified, officials said. "Biological testing continues on other workers who were at the site following the initial exposure event, and there is always the possibility of positive exposure results from that testing," said Farok Sharif, president of Nuclear Waste Partnership LLC, the contractor that runs WIPP. "The ability to detect extremely small amounts of radioactive material also means there may be false positives that occur through the testing process," he said. DOE officials also said ongoing monitoring has turned up no significant off-site contamination. The U.S. Environmental Protection Agency backed up those findings Wednesday in a letter to members of New Mexico's congressional delegation. Ron Curry, the EPA's regional administrator and the former head of New Mexico's Environment Department, told the lawmakers that any public health concern would be unlikely given the initial field measurements and modeling. WIPP is the nation's first underground nuclear repository and the only facility in the country that can store plutonium-contaminated clothing and tools from Los Alamos National Laboratory and other federal nuclear sites. It has been closed since Feb. 5, when a truck hauling salt through the underground mine caught fire. Several days later, sensors alerted to the release of radiation that exposed the 13 workers who were on the surface. Monitors as far as half a mile away detected elevated levels of plutonium and americium in the air.