LOS ALAMOS, N.M. — Officials say crews have made their first trip into the federal government's underground nuclear waste dump since a release of radiation in February contaminated 21 workers.

The U.S. Department of Energy says two crews of eight made the initial descent into the half-mile deep Waste Isolation Pilot Plant near Carlsbad in southeastern New Mexico, and no radiation was detected.

The agency called the entries a critical first step toward figuring out what caused the leak. But they say more expanded trips will be needed to continue the probe and assess the extent of damage.

Shipments to the dump were halted Feb. 5 after a truck hauling salt in the mine and repository caught fire. The dump was shuttered nine days later after the mysterious leak sent low levels of radiation into the air.