Entrance to Fort Hood
Entrance to Fort HoodReuters

Four people, including the shooter, have been killed and 14 injured after a gunman opened fire Wednesday afternoon (local time) at Fort Hood, the large military post in Texas, reports the New York Daily News.

Earlier reports said there were two shooters involved, but now authorities are saying that there was one shooter involved, and that he died of a self-inflicted gunshot wound.

KVUE television out of Austin, Texas, reported earlier that some 20 rounds had been shot outside at around 4:35 p.m. local time, before the shooter went in to the medical brigade building.

A soldier on post at Fort Hood said there appeared to be some sort of standoff happening.

Fort Hood's official account tweeted, "All personnel on post are asked to shelter in place."

Waco police said emergency sirens were sounding on post, and personnel were being advised to stay away from windows and keep doors closed and locked.

"Injuries are being reported. Be AWARE!! If you are in the Ft. Hood Post area there is an on-going active shooter," the department tweeted.

Fort Hood was the scene of a deadly 2009 shooting rampage, when Army Major Nidal Hasan opened fire at the base, killing 13 and wounding 32.

Hasan was sentenced to death for the attack last August. Hasan admitted to the massacre, claiming that the U.S. was waging a "war on Islam" and that he had "switched sides," becoming a devout Muslim and exchanging emails with notorious Al-Qaeda cleric Anwar al-Awlaki in the run-up to the attack.