The attorneys for James Holmes said they weren't ready to enter a plea for him on Tuesday, so a judge entered a standard not guilty plea on his behalf.
Holmes is charged with a total of 166 counts of murder, attempted murder and other charges related to the mass shooting at an Aurora, Colorado movie theater in July 2012 that left 12 people dead and 58 injured.
Holmes' attorneys had suggested they might enter a plea of not guilty by reason of insanity, according to court documents.
The judge said Holmes could be subjected to 'truth serum' and a polygraph to determine if he really was legally insane at the time of the shootings.
The attorneys will have to get the judge's approval if they still want to enter an insanity plea.
Prosecutors will decide on whether or not to seek the death penalty against the 25-year-old at a hearing on April 1.
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