Investigators are working to piece together the gunman's motive and plan.
Wearing a military uniform and carrying a semiautomatic pistol and another handgun, the attacker opened fire Thursday afternoon inside a medical processing center where soldiers returning home or about to be sent overseas were waiting to see doctors.
The soldiers do not regularly carry guns on Fort Hood, and so were not armed and able to respond to the gunman. Civilian police officer Kimberly Munley, who according to army officials arrived with her partner within three minutes of reports of gunfire, shot Maj. Nidal Malik Hasan, 39, and put an end to the rampage. Munson was also wounded in the shooting.
During remarks at the White House Friday morning, President Barack Obama says the "whole nation is grieving right now" over the mass killings at Fort Hood. He alsourged people not to jump to conclusions while law enforcement officials gather facts about the shootings.
A woman died overnight from her wounds, raising the toll from Thursday's incident to 13 dead and 30 wounded, said Colonel John Rossi, a spokesman at Fort Hood.
Witnesses said in media interviews that medics at the center tore the clothing of the dead and wounded to make tourniquets and administer first aid.
Much remains unknown about the motive, as Army officials have not been able to question Hasan due to his medical condition. Army spokesman Col. John Rossi declined to speculate as to the motive at a news conference Friday.
Hasan, an American of Jordanian descent, was born in Arlington, Va., and raised there and in Roanoke, Va. He has been described as a devout Muslim, according to the Washington Post.
He received his medical degree from the Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001. He had worked at Walter Reed Army Medical Center treating soldiers returning from Iraq and Afghanistan for six years. But an anonymous official told the Associated Press that Hasan received a poor performance evaluation at Walter Reed. And while he was an intern he had some "difficulties" that required counseling and extra supervision, Dr. Thomas Grieger, who was the training director at the time, told the AP.
Some sources are reporting that Hasan was opposed to the U.S. involvement in Iraq and Afghanistan, and was upset about recent news that he was to be sent overseas.
In a statement issued by Nader Hasan, a cousin of Nidal Hasan, his family said "we are filled with grief for the families of today's victims. Our family loves America. We are proud of our country, and saddened by today's tragedy."
Hasan's relatives in Northern Virginia later issued another statement calling the actions of their relative "despicable and deplorable."
American Muslim groups issued statements expressing regret over the incident and emphasizing that it appeared to have been carried out by a single disturbed individual.
"Thousands of Arab Americans and American Muslims serve honorably everyday in all four branches of the U.S. military and in the National Guard," the Arab American Institute said, according to news agencies.
Kim Fuller, a spokeswoman for Hasan's family says relatives are reaching out to law enforcement to offer insight. His family said in a statement Friday that his actions don't reflect how they were raised in the U.S.
Officials are investigating how the gunman brought a weapon onto the base, where soldiers do not regularly carry guns but are allowed to own privately registered weapons according to local laws.
"He could have just brought it onto the installation," Col. Rossi said, according to the New York Times.
A widespread investigation by the Army, FBI and other agencies began Thursday night, and the Department of Homeland Security said the Army would take the lead in the investigation. FBI agents are questioning residents of a townhouse complex in Kensington, Md. Where Hasan used to live, and were investigating his home in Killeen, Texas on Friday morning.
Fort Hood, near Killeen, is about 100 miles south of Dallas-Fort Worth, is the largest active duty military post in the U.S. Officials there declared a day of mourning Friday.
Reporter Erica Aguilar of KUT public radio in Austin, Texas, said that security is still tight on the base.
"The lockdown has been lifted but you've got long lines at all the gates trying to get in, because they're making sure everyone is checked," she said.
Listen to Aguilar describe the scene at Fort Hood:
---- Compiled from wire reports and other media sources