FORT HOOD, Texas (KWTX) -- An Iraq war veteran undergoing diagnosis for post-traumatic stress disorder opened fire late Wednesday afternoon on post, killing three soldiers and injuring 16 others before taking his own life as he was confronted by a female military police officer, Fort Hood and III Corps Commander Lt. Gen Mark Milley said Wednesday night.

He declined to identify the soldier, who he said arrived at Fort Hood in February.

The soldier, who was assigned to the post's 13th Sustainment Command, was receiving mental health treatment and was on medication.

Other sources, however, identified the gunman as Ivan Lopez, 34.

Lopez, an Army truck driver, was in uniform at the time of the shooting.

He was armed with a .45 caliber Smith and Wesson automatic pistol that he recently purchased locally, Milley said.

The weapon was not registered on base as required, Milley said.

He opened fire first in a building in the 1st Medical Brigade area, Milley said, and then got into a vehicle, firing several shots he drove to a second building in the 49th Transportation Battalion area.

He got out, entered the second building, and opened fire again, Milley said.

The female military police officer confronted him in the parking lot and shot him as he pulled out his gun, Milley said.

There's no indication the shooting was terrorism-related, Milley said.

Federal, state, military and local authorities are involved in the investigation of the shooting, he said.

Some of the wounded were taken to Carl R. Darnall Army Medical Center on post and others to Scott & White Hospital in Temple.

Scott & White officials said during a news conference Wednesday night that four patients were being treated at the hospital and two more were en route.

The hospital was notified at 5:05 p.m. of the shooting, officials said.

The victims, who were transported by either helicopter or ambulance, suffered gunshot wounds, some in the extremities, others in the abdomen, chest, and neck.

Some were shot repeatedly, officials said.

Their conditions ranged from stable to "quite critical."

The shooting occurred at around 4:25 p.m. in the area of a motor pool near Motor Pool Road and Tank Destroyer Boulevard.

A man who said he was a witness told News 10 that about 20 shots were fired.

"There has been a shooting at Fort Hood and injuries are reported. Emergency crews are on the scene. No further details are known at this time," the post said in a brief statement issued just after 5:30 p.m. Wednesday.

Warning sirens sounded late Wednesday afternoon at Fort Hood because of the incident.

People on post were told to stay indoors.

A message that scrolled across the top of the post's website said, "Shelter in place immediately. This is not a test."

The 1st Calvary Division, which is based at Fort Hood, sent a Twitter alert telling people on base to close doors and stay away from windows.

Killeen ISD schools were holding students until parents arrived because of the lockdown on post.

Campus cafeterias are open to the students and administrators will remain until the last of the children are picked up.

Texas A&M Central Texas in Killeen canceled evening and night classes Wednesday at Fort Hood and at its Fairway building because of the situation on post.

First responders from surrounding communities also headed to post.

Bell County sheriff's deputies and Department Public Safety troopers also responded, sheriff's Lt. Donnie Adams said.

A DPS helicopter was seen hovering over a car on the post, but it's not clear whether the vehicle was related to the shooting.

FBI agents were also reported to be en route, but the FBI did not immediately confirm that.

Media were being directed to the post's Visitor's Center.

On Nov. 5, 2009, Army psychiatrist Nidal Malik Hasan opened fire at Fort Hood's Soldier Readiness Center, killing 12 soldiers and one civilian and wounding 29 others before two Fort Hood civilian police officers shot him.

He is now on the military's death row.

President Barack Obama said the government will get to the bottom of what happened.

VIDEO: Obama 'heartbroken' over Ft. Hood shooting


Mr. Obama said he's following the situation closely, but said that the situation is fluid.

He said officials are doing everything they can to make sure everyone is secure.

Mr. Obama said the incident brings back painful memories of the 2009 mass shooting at Fort Hood.

"We're heartbroken that something like this might've happened again."

"But just for now I would hope that everyone across the country keep the families of Fort Hood in our thoughts and our prayers. The folks there have sacrificed so much on behalf of our freedom. Many of the people there have been on multiple tours of Iraq and Afghanistan, they served with valor, they served with distinction. At their home base they need to feel safe. We don't yet know what happened tonight but obviously that sense of safety has been broken once again. We need to find out exactly what happened.," he said.

Mr. Obama spoke at a restaurant in Chicago where he held a fundraiser.

Defense Secretary Chuck Hagel called the shooting a "terrible tragedy."

"There is nothing more important to us as an institution than the safety and well-being of our people, and for that reason I am grateful to all the first responders who rushed to the scene. We will closely monitor the situation at Fort Hood and stay informed by what investigators and law enforcement personnel learn about the shooting," he said.

"Tonight, Texans hearts are once again very heavy," U.S. Sen. John Cornyn, R-Texas, said in a statement.

"The scenes coming from Ft. Hood today are sadly too familiar and still too fresh in our memories. No community should have to go through this horrific violence once, let alone twice. I ask that all Americans join Sandy and me in praying for the victims, their families and the entire Ft. Hood community."

Gov. Rick Perry issued a statement Wednesday night in which he said, "Today Fort 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. The thoughts and prayers of all Texans are with everyone affected by this tragedy," he said.

State Sen. Troy Fraser, R-Horseshoe Bay, whose district includes the post, issued a statement asking for prayers "for peace and healing."

"Tonight, I join countless others in grieving following the senseless violence at Fort Hood. My thoughts and prayers are with all the brave men and women in uniform who serve us at Fort Hood and with their families and loved ones," he said.