President Obama called Thursday's deadly shooting rampage at Fort Hood a "horrific outburst of violence" and said his "immediate thoughts and prayers" are with the wounded and the families of those who died. 

The U.S. Army says 12 people have been killed and 31 wounded in the shooting. 

The president said he is working with the Pentagon, the FBI and the Department of Homeland Security to ensure security at the Texas military post. 

"It's difficult enough when we lose these brave Americans in battles overseas. It's horrifying that they should come under fire at an Army base on American soil," Obama said. "We will make sure that we get answers to every single question about this horrible incident. ... We are going to stay on this."

The House of Representatives and the Senate observed a moment of silence late Thursday afternoon for the victims of the attack.  

Vice President Biden also issued a written statement expressing his condolences. 

"We are all praying for those who were wounded and hoping for their full and speedy recovery. Our thoughts and prayers are also with the entire Fort Hood community as they deal with this senseless tragedy," he said. 

The shooting began around 1:30 p.m.  Lt. Gen. Bob Cone says that all the casualties took place at the base's Soldier Readiness Center where soldiers who are about to be deployed or who are returning undergo medical screening. 

He said the primary shooter used two handguns in the attack. 

Obama received a phone briefing on the incident from Cone late Thursday. During the call, the president expressed his condolences to Cone and all base personnel. 

The Associated Press contributed to this report.