In a letter sent to Judge Carlos Samour Wednesday and made public Thursday, institute superintendent William May said the psychiatrist conducting the evaluation wanted an extension from the end of July until Sept. 16, The Denver Post reported.
May said the psychiatrist was "reviewing an extraordinary amount of documentation."
"The evaluator has established a tentative plan for completion of the examination, which will likely require testing, interviews and/or assessments by a psychologist and neuropsychologist," May wrote.
Holmes pleaded not guilty by reason of insanity, prompting Samour to order the defendant undergo a psychiatric evaluation at the institute in Pueblo.
The judge issued an order telling both sides to choose a date next week for a hearing to discuss the request, the Post said. Samour said the request could impact motions hearings scheduled for August, as well as the trial date set for February.
Holmes faces the death penalty if convicted of murder for the attack on the Century Aurora 16 movie theater, in which 12 people were shot and killed and another 58 wounded.