Christian Soto appeared in court via video on Monday.
A judge granted a motion to determine if Soto is mentally fit to stand trial.
Police say last month, Soto killed four people. Seven others were hurt.
A motive is unclear, but Soto told investigators he was high on drugs during the attack.
SEE ALSO | Rockford stabbing suspect Christian Soto ordered to stay in jail until trial, shouted at judge