RJ Sangosti/The Denver Post/Getty
07/25/2012 AT 03:40 PM EDT
James Holmes, the suspect in the Aurora movie theater shooting
that left 12 dead and 58 injured, mailed planned details of his rampage to a University of Colorado psychiatrist before the July 20 attack, Fox News reports
But according to a law enforcement source, the unopened package sat in a mailroom for as long as a week before it was discovered on Monday, July 23.
Police and FBI agents were called to the University of Colorado Anschutz Medical Campus on Monday after the psychiatrist (who is also a professor at the school) reported receiving a package believed to be from Holmes.
Although that initial package proved to be harmless, a search turned up another package – this one reported to be from Holmes. Upon opening it, a notebook documenting the chilling details of Holmes's planned attack was discovered, according to the Fox report.
"There were drawings of what he was going to do in it – drawings and illustrations of the massacre," the source told Fox News.
The source added that the package had been in the mailroom since July 12, but it is unclear why it had not been delivered to the psychiatrist earlier.
It is also unknown whether the psychiatrist had any prior contact with Holmes.
For more on the tragedy in Aurora, including exclusive interviews with survivors and one trauma surgeon's harrowing account of her night in the E.R., pick up the new issue of PEOPLE, on newsstands Friday