UPDATE/FORT HOOD, TX - A senior officer at Fort Hood, Texas, says the soldier accused of opening fire on fellow service members Wednesday had been evaluated before the shooting to determine whether he had post-traumatic stress disorder.
He says Ivan Lopez served in Iraq in 2011 and had sought help for depression, anxiety and other problems. On Wednesday, he allegedly shot and killed three people and wounded or injured 16 others before killing himself.
NBCNews.com - Four people were dead, including the gunman, and at least six others were wounded in a shooting at Fort Hood, Texas, military officials told NBC News.
The gunman, identified as Ivan Lopez, 34, took his own life, officials said. Two of his victims died in local hospitals, while two other people were described as in "extremely grave" condition.
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 event appeared to have stemmed from a personal dispute in 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."
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.
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.
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.
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.