Fort Hood has ordered everyone at the Texas Army base to "shelter in place."
The order was sent Wednesday on the base's Twitter feed and posted on its Facebook page.
The 1st Calvary Division, which is based at Fort Hood, sent a Twitter alert telling people on base to close doors and stay away from windows.
A spokeswoman for the base declined to comment. A spokesman for the local sheriff's office did not immediately have information on the incident.
The base was the scene of a mass shooting in 2009. Thirteen people were killed and more than 30 wounded in what was the deadliest attack on a domestic military installation in history.