At least seven people are dead and 20 wounded in a mass shooting Thursday at Fort Hood, Texas, and at least one suspect is believed to be holed up in a building and shooting at SWAT team members, NBC News and affiliate KCEN reported.

It was unknown whether the victims were all soldiers or civilians at Fort Hood, one of the largest military complexes in the world.

One gunman was reportedly in custody and another was on the loose, NBC News said. A third shooter may be involved, according to NBC News affiliate KCEN in Waco, which said the person was holed up in building 42006 on the base and had opened fire on SWAT team members. KCEN quoted a source as saying the shooter had a high-powered rifle.

KCEN reported that a policeman was among those shot.

Greg Schannep, an aide to U.S. Rep. John Carter, told the Austin American-Statesman that he was on the Army post to attend a graduation service. He said that as he neared the entrance of a building where the service was being held, a soldier with blood on his uniform ran past him and said a man was shooting.

Schannep told the newspaper that the shootings appeared to have occurred in a complex near a theater where the service was scheduled. He was with the injured soldier, who he said appeared to have been struck in the shoulder but did not have life-threatening injuries.

The shooting happened "near the soldier readiness processing center, where soldiers cycle through as they prepare to deploy," according to Sgt. Major Jamie Posten from the Fort Hood public affairs office.

He said this is where the soldiers go, up to 60 days before deployment, to begin the process of vaccinations and other preparation. That complex is on the West side of post, off Battalion Avenue.

"It's still developing. We're trying to figure out what took place on the ground," he said.

A nurse at Metroplex Hospital on Fort Hood confirms that those injured in the shooting were being taken there. She told NBC they were all military personnel. The base was reportedly on lockdown. The Killeen Independent School District said all Fort Hood schools are on lockdown.

Temple ISD is on a "soft" lockdown. Parents will be able to pick their children up at the normal times, though they may experience delays.

FBI agents were reportedly on the way to the post.

Fort Hood is adjacent to Killeen, and 60 miles northeast of Austin. The sprawling complex is home to at least 4,929 active duty officers and 45,414 enlisted. Civilian employees total nearly 9,000.

Army spokesman Lt. Col. Lee M. Packnett said he was unaware if security measures were put in place at other military bases.

“At this point I do not know,” Packnett said.

President Barack Obama was briefed on the shootings, according to spokesman Rober Gibbs.

Sen. Kay Bailey Hutchinson, R-Texas, reacted to the news: "This is an unspeakable horror for the families, and our hearts go out to them at this time," she told KXAS in Dallas.

Texas Gov. Rick Perry's campaign spokesperson said the governor was in Denton, scheduled to attend a campaign event, when word of the shooting occured. There was no word on whether he has left Denton or if he was headed to Fort Hood.

Fort Hood has seen other violence in recent years. In September 2008, a 21-year-old 1st Cavalry Division soldier shot his lieutenant to death and then killed himself. Spc. Jody Michael Wirawan of Eagle River, Alaska. shot himself to death after shooting 1st Lt. Robert Bartlett Fletcher, 24, of Jensen Beach, Fla. to death.