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A shooting at the Fort Hood military installation in Texas left at least four people dead, including the gunman, and more than a dozen were injured, according to authorities.

The gunman, identified by multiple government sources as Army Specialist Ivan Lopez, took his own life, officials said.

Lopez, 33, of Kileen, Tex., was wearing an Army uniform at the time of the shooting, Michael McCaul (R-Tex.), chairman of the House Homeland Security Committee, told reporters.

Four people were taken to Scott and White Memorial Hospital in Temple, Tex., and another two are being brought there, said Glen Couchman, the facility’s chief medical officer. Their injuries that “range from stable to quite critical,” he said.

The installation was locked down for much of the afternoon and into the evening after the shooting before being lifted shortly before 9 p.m. local time.

Speaking in Chicago, President Obama said his administration was following the shooting closely.

“I want to just assure all of us we are going to get to the bottom of exactly what happened,” he said. “We’re heartbroken something like this might have happened again.”

The base was the site of a shooting in 2009 that ultimately killed 13 people and wounded another 32, the worst mass murder at a military installation in U.S. history. Nidal Hasan was sentenced to death last year for the shooting after being found guilty of premeditated and attempted premeditated murder.

Two Pentagon officials said investigators were still trying to determine a motive. But they said there was no indication that the shooter harbored similar ideological or religious beliefs to Maj. Hasan, an American-born Muslim who had been in contact with an al-Qaeda leader in Yemen and shouted “Allahu Akhbar,” or “God is great,” as he opened fire.

“The scenes coming from Ft. Hood today are sadly too familiar and still too fresh in our memories,” Sen. John Cornyn (R-Tex.) said in a statement. “No community should have to go through this horrific violence once, let alone twice.”

Secretary of Defense Chuck Hagel called the Fort Hood shooting a “terrible tragedy.”

“We know there are casualties, both people killed and injured,” Hagel told reporters and defense officials aboard the USS Anchorage in Honolulu, where he is hosting defense ministers from Asian countries.

The shooting comes weeks after the Defense Department released the results of a series of probes into a mass shooting at the Navy Yard.

Hagel said the Pentagon still has work to do in protecting its personnel and bases on the home front.

“When we have these kinds of tragedies on our bases, something’s not working,” Hagel said. “We will continue to address the issue. Any time you lose your people to these kinds of tragedies, it’s an issue, it’s a problem.”

This post has been updated. First posted: 6:12 p.m. 

Craig Whitlock, Ernesto Londoño, Philip Rucker, Adam Goldman and Sari Horwitz contributed to this report.