The US shooter accused of killing 13 and injuring more than 30 at the Fort Hood Army base in Texas in 2009 admitted to committing the violence in front of a jury.
"The evidence will clearly show that I am the shooter," Major Nidal Malik Hasan, a 42-year-old army psychiatrist, told the jury Tuesday, Xinhua reported.
"I apologise for any mistakes in this endeavour," he said.
The American-born muslim wanted to argue that he shot the US troops to protect Taliban fighters in Afghanistan from the US assaults. But the judge forbade him from using that defence.
Hasan was interrogated before the jurors in a wheelchair as he was shot in the back by military police during the rampage and was paralysed from the waist down.
Hasan fired his lawyers and acted as his own attorney. He released several statements prior to the trial, explaining his motive and criticising the US government's "anti-Islam" policies.
He faces 13 charges of pre-meditated murder and 32 charges of attempted pre-meditated murder in the November 2009 attack at the Texas Army base.