Crews have made their first trip into an underground nuclear waste repository in the U.S. state of New Mexico where a radiation leak had contaminated 21 workers and shut down the facility for nearly seven weeks, U.S. media reported Thursday. Two crews of eight inspectors ventured into the Waste Isolation Pilot Plant near Carlsbad in southeastern New Mexico Wednesday and found no airborne radiation, local TV KOB quoted officials with the U.S. Department of Energy as saying. The inspectors, consisting of radiological and mine-safety experts, descended by elevator to the underground interior of the plant to establish a base of operations and measure radiation levels, spokeswoman for the Energy Department Carrie Meyer told the media. Officials called the re-entry a critical first step toward finding out what caused the leak. More expanded operations will be carried out in the upcoming weeks. The much-awaited re-entry came nearly seven weeks after the underground dump was closed on Feb. 14 when air sensors detected unusually high levels of radioactive particles. It has been kept closed following reports that small amount of radiation was detected both at the underground and surface levels. Previously 17 workers were confirmed positive for radiation. The Energy Department said Monday that four more workers were tested positive, though their levels of exposure were well below those deemed unsafe. The prolonged closure of the repository has forced Los Alamos National Laboratory, a nuclear weapons manufacturing facility in New Mexico which used to store its refuse at the plant, to relocate its radioactive waste to Texas. The cause of the radiation leak still remains unknown. A truck fire was reported at the underground site on Feb. 5 and prompted evacuations, but officials said the fire was in a different part of the site and did not seem to be related to the leak. The repository stores "transuranic waste" leftover from nuclear weapons research and testing from the country's past defense activities, according to the Energy Department website. The waste includes clothing, tools, rags and other debris contaminated with radioactive elements, largely plutonium.