A reported active shooter situation has ended, and police have given the "all clear" to lift a lock down at the Redstone Arsenal Army post outside of Huntsville, Alabama.

No arrests or injuries were reported when the lock down ended, after people there were initially told to "run, hide, fight" during the evolving scenario seen there.

Officials weren't immediately clear what had sparked the lock down. Redstone spokesperson Christopher Colster said that the lock down was not a drill, and that there weren't any injuries immediately reported. A lock down drill had been scheduled for Wednesday.

Gates to the complex were initially closed, however some were reopened throughout the day, until the lock down was lifted.

The facility has more than 40,000 employees, and the Redstone grounds cover around 30,000 acres. There are less than 1,500 uniformed employees at the facility.

The Department of Homeland Security, the FBI, and local agencies were reportedly responding to the events, and were present at the site.

The incident was reported to be around the facility's Sparkman Centre, which is the headquarters for the Aviation and Missile Command there.

Childcare facilities at Redstone had performed safety checks and indicated that all children were safe and accounted for on the grounds.