(CNN) -- Three people were killed and 16 injured Wednesday when a shooter opened fire at Fort Hood, the sprawling Army post in Texas still on edge after a mass shooting there left 13 dead in 2009, officials said.
The gunman also died. He was engaged by military police before he fatally shot himself in the head, said the Army post's commander, Lt. Gen. Mark A. Milley.
The suspect, a soldier who had served in Iraq, "had behavioral health and mental health" issues, Milley told reporters late Wednesday.
The general said there was no known motive for the shooting.
"There is no indication that this incident is related to terrorism, although we are not ruling anything out," he said.
The initial report is the incident started as a soldier-on-soldier attack, law enforcement sources told CNN.
Scott & White Hospital in Temple, Texas, was treating eight patients with another expected to arrive shortly, said Dr. Glen Couchman, the chief medical officer. Three patients were in critical condition, while "the remaining are all seriously injured," he said.
Other patients were taken to Darnall Army Medical Center, not far from the Medical Brigade building where the shooting occurred, according a soldier who lives nearby and spoke on the condition of anonymity.
The suspected shooter was wearing combat fatigues, said a U.S. official briefed on the shooting. He was armed with a .45-caliber Smith & Wesson handgun that he purchased recently, said Milley.
President Barack Obama was briefed and said Wednesday evening "we're heartbroken something like this might have happened again."
He was referring to the November 2009 massacre.
"I want to just assure all of us we are going to get to the bottom of exactly what happened," the President said.
Defense Secretary Chuck Hagel also was monitoring the situation. He described what happened as a "terrible tragedy."
Earlier, Fort Hood's official Twitter feed asked that all personnel on post shelter in place.
Sheriff's deputies from Bell County and state troopers assisted by securing the area around the post, according to Bell County Sheriff's Lt. Donnie Adams. Authorities in the town of Killeen, just outside the gates, were also standing by to help.
"We are very concerned. Fort Hood is always there for us and we want to be there for them," said Killeen Mayor Dan Corbin.
He continued: "They are used to dealing with combat situations, and I'm sure they are very capable of handling this."
The lockdown was lifted just prior to 9 p.m. (10 p.m. ET), military personnel at the front gate told CNN.
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.
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.
Wednesday's shooting reminded many in the central Texas community of that incident.
"Today, Ft. Hood was once again stricken by tragedy. As Texans, our first priority must be caring for the victims and their families. Ft. Hood has proven its resilience before, and will again. Texas will support those efforts in any way we can, with any resources necessary," said Gov. Rick Perry.
U.S. Sen. John Cornyn said in a statement that no community should have to experience such violence once, let alone twice.
"Tonight, Texans' hearts are once again very heavy. The scenes coming from Fort Hood today are sadly too familiar and still too fresh in our memories," he said.
According to the Fort Hood website, the post is one of the largest in the world with 45,414 assigned soldiers and 8,900 civilian employees.
The installation, which encompasses 214,000 acres, is home to two divisions -- the Army's 1st Calvary and the 4th Infantry (Mechanized). There are 12 other units attached or based there.
CNN's Steve Almasy, Nicole Dow, Greg Botelho, Evan Perez, Shimon Prokupecz, Pamela Brown, Brian Todd, Matt Smith, Barbara Starr, Carma Hassan and Devon Sayers contributed to this report.