This story is developing. It was updated most recently at 11:15 p.m. ET on April 2.

Highlights: 3 things you need to know now

A soldier opened fire at the Fort Hood Army complex in Texas on Wednesday.

Three members of the military were killed in the shooting, and 16 were injured. The shooter died of a self-inflicted gunshot wound.

Military officials say the shooter, who served in Iraq in 2011, was under treatment for mental and behavioral problems.

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A gunman opened fire on Wednesday at Fort Hood military base in Killeen, Texas — the same site of a November 2009 mass murder that claimed 13 lives.

Three people were killed in the shooting and 16 were injured. The shooter died of a self-inflicted gunshot wound.

Speaking to reporters late Wednesday evening, Fort Hood's Lt. Gen Mark Milley said there was no known motive in the shooting. He said the shooter, a soldier who served four months in Iraq in 2011, had mental health and behavioral health issues and was being treated for them. He had not been diagnosed for PTSD. The shooter, who is married and has a family, has not been identified pending next-of-kin notification. He used a .45 Smith & Wesson that he purchased in the local area, Lt. Gen Milley said.

The attack was stopped by a female MP, who was "clearly heroic….exactly what we would expect from a United State military police," he said.

President Obama, speaking in front of a black backdrop at a Chicago steakhouse, said, "We are going to get to the bottom of exactly what happened. Any shooting is troubling. Obviously this reopens the pain of what happened at Fort Hood five years ago."

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

Image: Carolyn Kaster/Associated Press

Defense Secretary Chuck Hagel, speaking to reporters on Wednesday evening, said, "It's a terrible tragedy, we know that. We know there are casualties, of people getting killed and injured."

Image: Alex Wong, Pool/Associated Press

The injured personnel were transported to Carl R. Darnall Medical Center, as well as other local hospitals.

Scott and White Memorial Hospital Chief Medical Officer Dr. Glenn Couchman said on Wednesday evening he was "notified by officials at Fort Hood that there had been a shooting incident over there" at about 5:05 p.m. local time. "We have four patients that are actually here now, and two more en route." Their injuries, he said, included wounds to the chest, abdomen, extremities, and neck, the official said. They ranged from stable to "quite critical."

Waco police had confirmed to Mashable on Wednesday afternoon that there was in fact an active shooter on the base. A spokesperson told us the police department has an official on scene at Fort Hood, and it was on lockdown.

Fort Hood consists of three sections: the main cantonment, West Fort Hood and North Fort Hood. It's one of the largest military installations in the world. Wednesday's shooting happened in the main cantonment, where the majority of the soldiers live and work.

"Around 20 rounds were shot outside but then the shooter went in to the medical brigade building, according to a soldier on post at Fort Hood," USA Today reported shortly after the news first broke of the shooting. "The soldier said there appeared to be some sort of standoff happening."

ALERT: All personnel on Fort Hood should shelter in place immediately, close doors and stay away from windows.

— 1st Cavalry Division (@1stCavalryDiv) April 2, 2014

Be AWARE!! If you are in the Ft. Hood Post area there is an on-going active shooter. Take shelter immediately and monitor media.

— WacoPolice (@WacoPolice) April 2, 2014

Fort Hood's website had a scrolling banner that was advising people on the base to shelter in place immediately. "This is not a test," it said at 6 p.m. ET.

Image: hood.army.mil

A witness shared a video on Instagram — which was later broadcast on CNN — that purported to show the active shooter warnings being broadcast on the base:

Military Police sought out the shooter as those on base sheltered in place.

Update: Reports indicate police looking for a Toyota in connection with #FortHood shooting. http://t.co/Y4u0hXuLoa pic.twitter.com/bPKEW1Wis8

— Houston Chronicle (@HoustonChron) April 2, 2014

Soldiers gathered around televisions — seen here at California's Fort Irwin — to watch the news about the shooting, which unfolded towards the end of the day as many were preparing to go home to their families.

Fort Hood soldiers watching the news from Fort Irwin. pic.twitter.com/YYE9mv4oUn

— Don Gomez (@dongomezjr) April 2, 2014

News of the shooting immediately brought to mind the deadly rampage on the same base on Nov. 5, 2009, when Maj. Nidal Hasan, who had been an Army psychiatrist, killed 13 people and wounded more than 30 others with a semiautomatic pistol.

Hasan, a Muslim, had been opposed to the wars in Iraq and Afghanistan. Prosecutors and Hasan himself said that he feared being deployed to a warzone and wanted to commit jihad to protect fellow Muslims from American troops.

In his military trial, Hasan chose to represent himself but did not offer a defense. He was convicted on all counts and sentenced to death in August 2013.

It's the second mass shooting on a military installation in less than a year. On Sept. 16, Navy veteran Aaron Alexis shot and killed 12 people inside the Washington Navy Yard, before being fatally shot by police.

The shooting comes one day after Fox News reported the FBI, and military were looking for an ex-Army recruit that is suspected of plotting "Ft. Hood-inspired jihad," in reference to the 2009 shooting. There have been no confirmed reports tying the suspected recruit to today's alleged shooting. Although authorities had not determined a motive for the shooting, Mr. McCaul said on Wednesday he was concerned that the base was “becoming a target for potential jihadists.”