Christopher Husbands, 25, has pleaded not guilty to two counts of first-degree murder in the June 2012 shooting.
His lawyers are expected to argue that he did not go to the landmark downtown mall with the intention to kill anyone.
Defence lawyer Dirk Derstine said in his opening arguments today that the case hinges on what his client was thinking at the time.
Husbands is expected to testify once the assessment is complete.
He is also charged with five counts of aggravated assault, one of criminal negligence causing bodily harm, and one of recklessly discharging a firearm.
He has pleaded not guilty to those charges as well.
Note to readers: This is a corrected story. An earlier version said Husbands is charged with two counts of second-degree murder.