CARLSBAD, N.M. —The Department of Energy says tests show four more workers were contaminated with low levels of radiation during a leak at federal government’s underground nuclear waste dump.

Officials also said Monday that they’re planning to get a crew underground Tuesday for the first time since the Valentine’s Day accident at the Waste Isolation Pilot Plant in southeastern New Mexico.

The DOE says a total of 21 workers received low doses of radiation, all well below levels deemed unsafe.

On Tuesday, the DOE said it plans to send a team of team of eight experts into the mine to begin setting up bases from which they can start investigating what caused the leak.

The dump is the nation’s only permanent underground repository for low-level radioactive waste from nuclear weapons facilities.