The incident at the Pantex Plant in Amarillo, Texas ended "without incident" less than an hour after it was first reported, according to its Twitter account.

Two police agencies were involved in what was described as a precautionary response to the incident.

The plant is operated by government contractors and is the nation's primary one of six facilities for the assembly and dismantlement of nuclear weapons.

A recording on the plant's phone system said it was operating normally.

An official with the National Nuclear Security Administration, which supervises the plant, declined to comment.