Photo: Pantex Plant, a USA nuclear weapons assembly and disassembly facility in Amarillo, Texas.
Its nuclear stockpile was in no danger and the lock-down was purely precautionary, a state safety official said.
According to Carson County Sheriff Tam Terry, the plant was locked down because armed hunters were spotted on property adjacent to it.
"Somebody saw some armed individuals dressed in camouflage clothing exiting the vicinity of the plant," said Terry.
Sheriffs found a pair of hunters setting out duck decoys and building a blind on property near the plant, Terry said.
"They were very cooperative and compliant," Terry said. "We identified them. We checked their criminal history."
"There was not a threat to the Pantex plant assets, workers, or the public, and the plant is now returning to normal operations," the company said in a statement.
The plant is operated by Babcock and Wilcox for the U.S. Department of Energy, which oversees the U.S. nuclear weapons stockpile.