Fort Hood 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.
Outside the base, some relatives of soldiers waited for news about their loved ones.
Tayra DeHart, 33, said she had last heard from her husband, a soldier at the post, that he was safe, but that was hours earlier.
"The last two hours have been the most nerve-wracking I've ever felt. I know God is here protecting me and all the soldiers, but I have my phone in my hand just hoping it will ring and it will be my husband," DeHart said.
In Chicago, President Barack Obama vowed that investigators will get to the bottom of the shooting, seeking to reassure the nation whose sense of security once again has been shaken by mass violence
In a hastily arranged statement, Obama said he and his team were following the situation closely but that details about what happened at the sprawling Army post were still fluid. He said the shooting brought back painful memories of the 2009, when 13 were killed at the same post in the deadliest attack on a domestic military installation in history.
Obama reflected on the sacrifices that troops stationed at Fort Hood have made — including during multiple tours to Iraq and Afghanistan.
"They serve with valor, they serve with distinction and when they're at their home base, they need to feel safe," Obama said. "We don't yet know what happened tonight, but obviously that sense of safety has been broken once again."
The president spoke without notes or prepared remarks in the same room of a steakhouse where he had just met with about 25 donors at a previously scheduled fundraiser for the Democratic National Committee. White House officials quickly pushed tables to the side of the room to make room for Obama to speak to the nation.
Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said many questions remained about the shooting and the focus was on support the victims and their families. "This is a community that has faced and overcome crises with resilience and strength," he added.
The November 2009 attack at Fort Hood 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 "Allah 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-barreled 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 13 people dead, including the gunman. After that shooting, Defense Secretary Chuck Hagel ordered the Pentagon to review security at all U.S. defense installations worldwide and examine the granting of security clearances that allow access to them.
Asked Wednesday about security improvements in the wake of other shootings at U.S. military bases, Hagel said, "Obviously when we have these kinds of tragedies on our bases, something's not working."
Associated Press reporters Lolita C. Baldor in Honolulu, Eric Tucker in Washington and Nedra Pickler in Chicago contributed to this report.