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James Holmes spelled out gory details of his planned massacre in a notebook which he mailed to a University of Colorado professor ... more than a week before he opened fire in the Aurora movie theater.

Police and FBI discovered the notebook while searching a mail room on the Aurora campus on Monday ... but it had reportedly been sitting there unopened since July 12 -- EIGHT days before the shootings that killed 12 people and injured 58.

The package was sent from Holmes, and according to a FOXNews.com source ... “Inside the package was a notebook full of details about how he was going to kill people. There were drawings of what he was going to do in it -- drawings and illustrations of the massacre."

The source says the drawings included gun-wielding stick figures shooting other stick figures.

The notebook was reportedly addressed to a professor/psychiatrist who treated patients at the campus' psychiatry facility. It's unknown if the professor ever had contact with Holmes.