Congress began reviewing the Fort Hood mass shooting that killed 13 and injured 31 others Thursday.

Several Congressional lawmakers called the Fort Hood attack an act of terrorism. Others say there is no sign the accused shooter, Maj. Nidal Hasan, worked with others.

An expert testifying at the hearing said Hasan has the characteristics of both a political extremist and an ordinary mass murderer.

"At a glance, Maj. Hasan's rampage at Fort Hood looks like what used to be called "going postal"— a deepening sense of personal grievance culminating in homicidal rampage directed against coworkers, in this case, fellow soldiers," RAND Corporation Senior Adviser Brian Jenkins said. "For Hasan, "going jihad" reflects channeling of obvious personality problems into a deadly fanaticism."

Jenkins went on to say there is often a murky distinction between terrorism and personal distress as a motivator.