The gunman, identified as Ivan Lopez, 34 — an enlisted soldier — took his own life, officials said.
Three other people died in local hospitals, they said.The conditions of the 11 people who were wounded were unreported, but military officials said one was in "extremely grave" condition.
Doctors at Scott & White Memorial Hospital said they were treating four patients, some stable and others in "quite critical" condition. Two more patients were en route and were expected to be taken directly to surgery for what were described as very serious injuries.
Dr. Harry Papaconstantinou told reporters the victims' injuries included gunshots to their extremities, abdomens, chests and necks.
Others were being treated at Carl R. Darnall Army Medical Center on base.
"This was another sad day for central Texas," said Dr. Glen Couchman, the hospital's chief medical officer.
Fort Hood is the same base where a military psychiatrist who proclaimed jihad against the U.S. killed 13 people 4½ years ago.
But military officials told NBC News the shooting appeared to have stemmed from a personal dispute at a motor pool and was unrelated to terrorism.
President Barack Obama spoke after the shooting, saying, "We're heartbroken something like this might have happened again."
He added, "We're following it closely. ... I want to just assure all of us we are going to get to the bottom of exactly what happened."
Defense Secretary Chuck Hagel, traveling in Hawaii, told reporters he had little information, but called the event a "terrible tragedy."
Witnesses and military officials said the shooting occurred about 4:30 p.m. (5:30 p.m. ET). The base's emergency alert system immediately sounded, and all personnel were told to shelter in place. The lockdown was lifted at around 7:30 p.m. (8:30 p.m. ET)
Few details were immediately available, but a supervisor at Scott & White Memorial Hospital told NBC News that the hospital was "setting up a command center."
Bell County and state public safety deputies were securing the perimeter of the area, a senior local law enforcement source said.
The FBI was also on scene to support law enforcement, according to the sources.
Waco police told the public to avoid the fort, saying on Twitter that "there is an on-going active shooter."
Nevertheless, dozens of friends and relatives of Fort Hood personnel gathered in the base's visitors' center seeking information about loved ones, NBC station KCEN of Waco reported.
Antonio Ortiz, 30, who lives a quarter of a mile from the east gate of Fort Hood, said he heard a commotion and went outside to hear alarms going off and announcements for people to stay inside.
He went back in and turned on the TV news, then soon after heard a barrage of gunshots.
"It sounded powerful,” he said, adding that while it seemed to be coming from the base, he couldn't rule out the possibility someone in the civilian neighborhood was shooting.
"I’m scared for my son. He’s 7," Ortiz said. "But I do have a 12-gauge pump shotgun."
Central Texas College nearby was being evacuated, and all Thursday evening classes at the college and at Fort Hood were canceled, the college said.
And several lawmakers — in and out of Texas — called for prayers afterwards.
Maj. Nidal Hasan was convicted in August of the killing in November 2009 and injured 32 others. He has since been sentenced to death.
In September, a gunman opened fire at the Washington Navy Yard, killing 12 and wounding four before being slain by police. Last month, a civilian shot dead a sailor aboard a ship at a U.S. Navy base in Norfolk, Va.
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