US Defense Secretary Robert Gates announced a thorough investigation into the Fort Hood shootings to examine if warning signs were missed and to ensure such a rampage never happens again.
US Army psychiatrist Major Nidal Hasan has been charged with the murder of 13 people in the November 5 attack at the military base in Texas, in which 42 people were also wounded.
Hasan is being investigated for links to Islamic extremism, including his contacts with a radical cleric who blessed the killing spree as an acceptable form of jihad, or holy war.
Gates asked former army secretary Togo West and former chief of naval operations Admiral Vernon Clark to lead a probe into possible "lapses or problems" that had raised "serious questions."
"We do not enter this process with any preconceived notions," Gates, who attended a funeral for one of the shooting victims on Wednesday, told a press conference yesterday.
"However, it is prudent to determine immediately whether there are internal weaknesses or procedural shortcomings in the department that could make us vulnerable in the future."
The Fort Hood shootings stunned a military already under severe strain from years of war in both Iraq and Afghanistan, with the army reporting a rise in depression, suicides and marital problems.