RENO, Nev. (AP) — The U.S. Energy Department says the weapons-grade plutonium it shipped from South Carolina to a security site near Las Vegas was the same type of material it has safely transported between its sites for decades.

The department said in a statement Thursday its National Nuclear Security Administration routinely ships similar radioactive material between its sites as part of its national security missions.

The agency disclosed for the first time on Wednesday that it trucked about 1,000 pounds of the plutonium to Nevada sometime between August and November when Nevada filed a lawsuit seeking to block the shipments.

The department said it had to keep the shipment secret until now for national security reason.