The package was sent to the University of Colorado's Anschutz Medical Campus and prompted a two-and-a-half hour evacuation of the building.
Initial media reports said it was delivered nearly two weeks ago, but on Wednesday evening, university officials confirmed the package arrived after the Aurora theater shooting.
"The suspicious package discovered at the Facilities Services building on Monday, July 23, 2012, was delivered to the campus by the United States Postal Service that same day, immediately investigated and turned over to authorities within hours of delivery," said Jacque Montgomery, a university spokesperson.
Investigators have not revealed what was in the package, but ABC News has learned it contained a notebook believed to be written by Holmes, who had just dropped out of the university last month.
FOX News is quoting a law enforcement source as saying the notebook contained "full details about how he was going to kill people ... drawings of what he was going to do in it, drawings and illustrations of the massacre."
Over the past year, Holmes began to struggle with poor grades in the university's PhD program. ABC News has learned that on June 7, after failing a key oral exam, Holmes went to a sporting goods store and bought a high-powered rifle. Three days later, he withdrew from the PhD program.
Holmes is accused of killing 12 people and wounding 58 more at a midnight showing of "The Dark Knight Rises" last Friday.
Five hospitals are still caring for 17 of the shooting victims, six of whom are in critical condition.
Meantime, members of Holmes' legal defense team visited the suspect's apartment in Aurora on Wednesday. They pored over the scene that -- just days ago -- was rigged to explode, laden with trip wires, homemade grenades and 10 gallons of gasoline. Residents of the apartment building were finally allowed to return home after being evacuated for five days.