Major Nidal Malik Hasan is accused of opening fire with two handguns on Nov 5 at a Fort Hood facility for preparing troops for overseas deployment.
"We are doing everything possible and we are looking at every reason for this shooting," said Christopher Grey, a spokesman for the Army's criminal investigation division.
"We are aggressively following every possible lead," Grey said.
The shooting spree left 13 soldiers and a civilian employee at the base dead. Forty-three others were wounded.
Hasan suffered gunshot wounds from two police officers who are credited with ending the rampage. Hasan remained hospitalised and was expected to recover.
He has been informed of the charges and obtained legal representation, Grey said.
President Barack Obama on Friday , Nov 13, ordered the military and other government agencies to gather any documents about Hasan prior to the attack to determine if there were signs he could turn violent.