Delivering closing arguments Wednesday, Stead said the defense expert, Dr. Mark Mills, was taken in by McCoy's lies and therefore could not fully evaluate the defendant.
Stead said the prosecution expert, Dr. Phillip Resnick, was more thorough in his evaluation.
In previous testimony, Resnick said he reviewed police and medical records before interviewing McCoy, who has pleaded not guilty by reason of insanity to 24 charges related to a five-month spree of 12 shootings.
Mills had said he interviewed McCoy before reviewing other records.