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The gunman in the Aurora theater shooting was a quiet, knowledgeable student who had a "cocky" attitude, one of his graduate school professors testified Wednesday.

As a student at the University of Colorado's neuroscience program, James Holmes only spoke when called upon, but always knew the answers, said assistant professor Achim Klug.

Klug had interviewed Holmes for admittance into the neuroscience program and did not think he was interested in attending because he had an offer from another institution, which Klug felt explained his "cocky" attitude.

Despite being "impressive on paper," Holmes was a weak interviewee, Klug said, and he suggested to the admissions committee that Holmes be held as an alternate for the program. Holmes was admitted nonetheless, and spent a year in the elite program before he dropped out. The shooting happened barely a month after he left.

Holmes, 27, has confessed to the July 20, 2012 movie theater shooting but has pleaded not guilty by reason of insanity. He is accused of killing 12 people and injuring 70 others during a midnight screening of the Batman movie "The Dark Knight Rises" inside an auditorium at Aurora's Century 16 movie theater.

Holmes was highly intelligent, Klug said, but he didn't put any effort into his lab work until the end of the program — when he suddenly displayed a "burst of initiative." Another professor who testified on Wednesday, Mark Dell'Acqua, also said Holmes was introverted and lacked motivation.

Holmes swiveled in his chair as he listened to his professors describe his shortcomings.

Jurors in the trial heard from survivors and first responders last week, who described the gory, chaotic scene at the movie theater. Earlier this week, witnesses testified about the explosive devices that Holmes had left inside his booby-trapped apartment the day of the shooting.

IN-DEPTH

— Elizabeth Chuck and Vicky Collins