FORT HOOD (CBSDFW.COM/AP) – A soldier opened fire Wednesday on fellow service members at the Fort Hood military base, killing three people and wounding 16 before committing suicide at the same post where more than a dozen people were slain in a 2009 attack, authorities said.

The shooter, who served in Iraq in 2011, had been undergoing an assessment to determine whether he had post-traumatic stress disorder, according to Lt. Gen. Mark A. Milley, the senior officer on the base. He was also undergoing psychiatric treatment for depression.

There was no indication the attack was related to terrorism, Milley said “although we are not ruling anything out.”

A Texas congressman said the shooting happened at a medical center. Rep. Michael McCaul, chairman of the House Homeland Security Committee, also identified the suspect as Ivan Lopez. But additional details about the gunman were not immediately available.

The injured were taken to Darnall Army Community Hospital at Fort Hood and other local hospitals. Dr. Glen Couchman, chief medical officer at Scott and White Hospital in Temple, said the first four people admitted there had gunshots to chest, abdomen, neck and extremities and that their conditions range from stable to “quite critical.”  At last check, nine patients had been admitted to Scott and White.

The 2009 assault on Fort Hood was the deadliest attack on a domestic military installation in U.S. history. Thirteen people were killed and more than 30 wounded.

After the shooting began, the Army’s official Twitter feed said the post had been locked down. Hours later, all-clear sirens sounded.

On Wednesday evening, a fatigue-clad soldier and a military police officer stood about a quarter-mile from the main gate waving away traffic. Other lanes were blocked by a police car and van.

Meanwhile, relatives of soldiers waited for news about their loved ones.

Tayra DeHart, 33, said she had last heard from her husband, a soldier at the post, that he was safe, but that was hours earlier.

“The last two hours have been the most nerve-racking I’ve ever felt. I know God is here protecting me and all the soldiers, but I have my phone in my hand just hoping it will ring and it will be my husband,” DeHart said.

Brooke Conover, whose husband was on base at the time of the shooting, said she found out about it while checking Facebook. She said she called her husband, Staff Sgt. Sean Conover, immediately to make sure he was OK, but he could not even tell her exactly what was going on, only that the base was locked down.

“I’m still hearing conflicting stories about what happened and where the shooting was exactly,” Conover said in a telephone interview, explaining that she still did not know how close the incident was to her husband.

“I just want him to come home,” said Conover, who moved to Fort Hood with her husband and three daughters two years ago.

President Barack Obama vowed that investigators would get to the bottom of the shooting.

In a hastily arranged statement in Chicago, Obama said he was following the situation closely. He said the shooting brought back painful memories of the 2009 attack.

Obama reflected on the sacrifices that troops stationed at Fort Hood have made – including enduring multiple tours to Iraq and Afghanistan.

“They serve with valor. They serve with distinction, and when they’re at their home base, they need to feel safe,” Obama said. “We don’t yet know what happened tonight, but obviously that sense of safety has been broken once again.”