Alleged movie theater gunman James Holmes mailed a notebook "full of details about how he was going to kill people" to a University of Colorado psychiatrist before the attack, but it sat unopened in a mail room until Monday, Fox News reported.

According to Fox, police and FBI agents were called to the University of Colorado Anschutz medical campus in Aurora on Monday after the psychiatrist said he received a package he thought was from Holmes. While that package turned out to be from someone else, a search of the mail room netted another package for the psychiatrist with Holmes' name in the return address.

“Inside the package was a notebook full of details about how he was going to kill people,” a law enforcement source told Fox. “There were drawings of what he was going to do in it - drawings and illustrations of the massacre."

According to Fox, "among the images shown in the spiral-bound notebook’s pages were gun-wielding stick figures blowing away other stick figures."

One source said the package had been in the mail room since July 12, eight days before the shooting.

The notebook is now in FBI possession. The agency declined to comment on the matter, citing a gag order on the case.

According to Fox, the psychiatrist the package was meant for is a professor at the school who treated patients at a psychiatry outpatient facility. It was not clear whether the psychiatrist had ever had contact Holmes, who dropped out of the school's neuroscience doctoral program last month.

Twelve people were killed and 58 others injured when Holmes allegedly opened fire during a midnight screening of the new Batman film, "The Dark Knight Rises." Holmes appeared in court for the first time on Monday; charges against him are expected to be filed July 30.