The verdict Friday against 42-year-old Major Nidal Hasan was handed down by a US military jury after a court martial at Fort Hood, the site of his 2009 shooting spree.
Hasan sat stoically as the foreman, a female colonel, read the verdict in the killings of 12 service members and a civilian, as well as the attempted murder of dozens more.
There were no outbursts of emotion from family members of the victims, but some cried and wiped away tears as they left the courtroom.
"So overwhelmed with joy and tears! ... God Bless the victims in their strength," former police sergeant Kimberly Munley, who was wounded in the shooting, wrote on Twitter.
The trial had heard that Hasan, a Muslim, had been in contact with a leading Al-Qaeda figure and had attacked his comrades out of opposition to the wars in Iraq and Afghanistan.
Hasan, who was serving at the time as an Army psychiatrist, chose to defend himself during the hearings and openly declared that he had carried out the killings. The trial will now move into a sentencing phase.
His refusal to engage with the court -- he called no witnesses and refused to make a closing statement -- has fuelled suspicions that he is actively seeking the death penalty.
On November 5, 2009, Hasan opened fire at a medical processing facility in the sprawling Fort Hood base that serves as a staging point for soldiers to deploy to combat zones.
Twelve of the dead and 30 of the wounded were soldiers. Hasan was himself shot by a civilian police officer who responded to the attack and he is now partially paralyzed.
Tim Hancock was mayor of the nearby town of Killeen at the time of the attack and 200 meters away attending a graduation ceremony when the first shots rang out.
"It was like war, in my mind," Hancock said. "It was like the real thing and it was the real thing for those in the processing center."
"I think justice is served and we'll see what happens from here. It is still hard to believe and still sad," he said of the sentencing.
Hasan's attack raised fears that the United States could face a wave of so-called "lone wolf" killers, inspired by Al-Qaeda but not directly under the extremist group's control.
"I was defending my religion," Hasan said in a letter to AFP, arguing that the United States was wrong to invade Muslim countries.
"It is one thing for the United States to say 'We don't want sharia law to govern us,' but it is not acceptable to have a foreign policy that tries to replace sharia law with a more secular form of government."
Under US military law, a full trial must be held in a death penalty case, even if the defendant wants to plead guilty.
But Hasan chose to defend himself and contested virtually none of the evidence presented during the two-week court martial.
At the outset, he declared: "The evidence will show that I am the shooter."
The military judge, Colonel Tara Osborn, told Hasan she thought it "unwise" for him to continue to serve as his own attorney as the case moves into the sentencing phase.
"I want to make sure your choice is made with your eyes wide open," Osborn said. "You're 42 years old, you're highly educated, you have a medical degree, but you are not legally trained."
Hasan told the court he had switched sides in the war on terror in order to battle US soldiers he believed were being sent to make war on Muslims.
Before the shooting, Hasan had learned he would be deployed to Afghanistan. He armed himself with two handguns before attacking the center.
Witnesses described chaotic scene in which dozens of soldiers were caught off guard.
When Hasan shouted "Allahu akbar" -- Arabic for "God is great" -- and opened fire, many believed it was a training exercise.
Hasan fired quickly, hitting some victims multiple times. One of the dead, specialist Frederick Greene, was hit 12 times while charging Hasan.
The shooting ended after civilian police confronted Hasan outside the building.
Police sergeant Munley was the first on the scene and opened fire. Hasan charged her and shot her three times.
Fellow officer Sergeant Mark Todd then arrived and opened fire on Hasan, hitting him several times.