A shooting at Fort Hood on Wednesday left four people dead, including the gunman, and 16 others injured, officials said.

The shooter, identified as 34-year-old soldier Ivan Lopez, died of a self-inflicted gunshot wound. Lopez was identified by Congressman Michael McCaul, chairman of the House Homeland Security Committee, in an interview with The Associated Press.

“We do not know a motive," Lt. Gen. Mark A. Milley told reporters at a press conference on Wednesday night. "But we know this soldier to have behavioral health and mental health issues.”

"In regards to the investigation, there is no indication this incident is related to terrorism, though we are not ruling anything out,” Milley added.

The shooter served four months in Iraq in 2011, Milley said, and "was undergoing treatment for depression, anxiety and a variety of other psychological and psychiatric issues.” The suspect had not been diagnosed with post-traumatic stress disorder, Milley stressed, but was being evaluated for PTSD.

The suspect used a .45-caliber Smith & Wesson semi-automatic pistol that had been purchased recently, Milley said.

Nine of the victims were transported to Scott & White Hospital with gunshot wounds, with three listed as being in critical condition. “Their conditions range from quite stable to quite critically injured," Glen Couchman, Scott & White's chief medical officer, said. Couchman said the hospital was not currently in need of blood donations from individuals. The other victims were reportedly transported to Carl R. Darnall Medical Center.

Milley said all of the shooting victims were members of the military.

Emergency crews, FBI and SWAT teams were called in to the base after the shooting, which occurred at approximately 4 p.m. at a medical support building on the sprawling base. Soldiers and area residents were ordered to shelter in place as police pursued reports of a possible second shooter. The lockdown was lifted several hours later.

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President Barack Obama said the White House was monitoring reports of the shooting.

"We're following it closely. The situation is fluid right now," Obama told reporters in Chicago, adding that investigators would "get to the bottom of exactly what happened."

"We're heartbroken something like this might have happened again," the president said.

In 2009, 13 people were killed and more than 30 wounded in a mass shooting at the base carried out by then-Maj. Nidal Hasan, an army psychiatrist. A U.S. Senate report after the incident described it as the worst attack on American soil since the terrorist attacks of Sept. 11, 2001, although the FBI said Hasan had no formal ties to terrorist groups despite having expressed anti-American viewpoints before the shooting.

Hasan was sentenced to death after admitting during his August 2013 court martial hearing to the mass shooting. He is now on death row.

"Ft. Hood was once again stricken by tragedy," Gov. Rick Perry said in a statement. "As Texans, our first priority must be caring for the victims and their families. Ft. Hood has proven its resilience before, and will again."

"The scenes coming from Ft. Hood today are sadly too familiar and still too fresh in our memories," Texas Sen. John Cornyn said in a statement on Wednesday. " No community should have to go through this horrific violence once, let alone twice."

Killeen, Texas, where Fort Hood is located, was also home to the then-deadliest mass shooting in U.S. history. Commonly known as "Luby’s Massacre," on Oct. 16, 1991 George Hennard shot 50 individuals, killing 23.

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