7:26 p.m. update: All-clear sirens have sounded at Fort Hood as the lockdown was lifted hours after a deadly shooting at the Texas Army post.
Hundreds of cars began streaming from the giant complex, including children who had been kept in locked-down schools.
The Texas Army base was the scene of a mass shooting in 2009.
U.S. law enforcement officials say four people, including the shooter, were killed when a gunman opened fire Wednesday at Fort Hood.
One of the officials, citing official internal U.S. Justice Department updates, said 14 others were hurt. The officials spoke on condition of anonymity because they were not authorized to release information by name.
Fort Hood, site of a 2009 mass shooting, said in a statement posted online that its Directorate of Emergency Services had an initial report that the shooter was dead, but that the report was unconfirmed. Additional details were not immediately available.
A U.S. law enforcement official said reports circulating within the Justice Department indicate the shooter died of what appears to be a self-inflicted wound. The official spoke on the condition of anonymity because the investigation is still ongoing.
The Army said on its official Twitter feed that the Texas Army post was still on lockdown. Injured people were being treated at the post's Carl R. Darnall Medical Center and other local hospitals.
President Barack Obama said he was following the situation closely and that the government will get to the bottom of what happened.
He said Wednesday's incident brought back painful memories of the massacre at the Texas Army post. Thirteen people were killed and more than 30 wounded in what was the deadliest attack on a domestic military installation in history.
"We're heartbroken that something like this might've happened again," Obama said at a restaurant in Chicago where he held a fundraiser.
"The folks there have sacrificed so much on behalf of our freedom. Many of the people there have been on multiple tours to Iraq and Afghanistan," Obama said. "They serve with valour, they serve with distinction and when they're at their home base, they need to feel safe. We don't yet know what happened tonight, but obviously that sense of safety has been broken once again."
The November 2009 attack happened inside a crowded building where soldiers were waiting to get vaccines and routine paperwork after recently returning from deployments or while preparing to go to Afghanistan and Iraq.
Army psychiatrist Nidal Hasan was convicted and sentenced to death last year in that mass shooting.
According to testimony during Hasan's trial last August, Hasan walked inside carrying two weapons and several loaded magazines, shouted "Allahu Akbar!" — Arabic for "God is great!" — and opened fire with a handgun.
Witnesses said he targeted soldiers as he walked through the building, leaving pools of blood, spent casings and dying soldiers on the floor. Photos of the scene were shown to the 13 officers on the military jury.
The rampage ended when Hasan was shot in the back by Fort Hood police officers outside the building, which left him paralyzed from the waist down. Hasan is now on death row at the military prison at Fort Leavenworth in Kansas.
After that shooting, the military tightened security at bases nationwide. Those measures included issuing security personnel long-barrelled weapons, adding an insider-attack scenario to their training, and strengthening ties to local law enforcement, according to Peter Daly, a vice-admiral who retired from the Navy in 2011. The military also joined an FBI intelligence-sharing program aimed at identifying terror threats.
In September, a former Navy man opened fire at the Washington Navy Yard, leaving at least 13 people dead, including the gunman. After that shooting, Hagel ordered the Pentagon to review security at all U.S. defence installations worldwide and examine the granting of security clearances that allow access to them.