There is an active shooter at the sprawling central Texas base of Fort Hood, a source at Fort Hood told CNN on Wednesday.
The situation is serious and sirens are going off, said the source, who spoke on the condition of anonymity.
A spokesman for the base said an "incident" was under way but declined for provide additional information.
Fort Hood's official Twitter feed asked that all personnel on post shelter in place.
A local sheriff's office said that it had heard there was a shooting.
"They got some kind of shooting incident going on. They haven't told us anything. It was on post and being handled by the military. Fort Hood has not asked for any assistance," said Lt. Donnie Adams with the Bell County Sheriff's Office.
Police in Waco, Texas, advised those on post to stay away from windows and to 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.
All evening and night classes for the Central and Fort Hood campuses of Central Texas College (CTC) were canceled, according to the school's website. Personnel and students were told to evacuate CTC Central Campus.
On November 5, 2009, Army Maj. Nidal Malik Hasan opened fire at Fort Hood, killing 13 people and injuring 32.
He shot fellow soldiers at the processing center. Prosecutors maintained that the American-born Muslim underwent a progressive radicalization that led to the massacre at the base.
Hasan allegedly picked that day because it was when the units he was scheduled to deploy with to Afghanistan were scheduled to go through the processing center.
The former Army psychiatrist was convicted of premeditated murder, and a military jury recommended that Hasan be put to death.