Sitting in a wheelchair, his voice soft but unwavering, U.S. Army psychiatrist Nidal Malik Hasan took responsibility Tuesday for the 2009 mass shooting at Fort Hood.

"Fellow members, good morning," Hasan began at the opening of his court-martial at the army post in central Texas. "On Nov. 5, 2009, 13 U.S. soldiers were killed and many more injured. The evidence will clearly show that I am the shooter."

Hasan, a 42-year-old American-born Muslim who is representing himself, was in a small courtroom just a few kilometers from Fort Hood's Soldier Readiness Processing Center, where he said he opened fire four years ago on fellow soldiers preparing to deploy to Afghanistan.