Authorities on Monday discovered a package in a mailroom at the University of Colorado - Anschutz Medical Campus apparently sent by the alleged gunman responsible for the theater shooting that left 12 people dead and scores injured, a law enforcement official briefed on the investigation said Wednesday.

It was sent to the school's campus in Aurora, where the suspect, James Holmes, had recently dropped out of a Ph.D. program in neuroscience.

CBS News reported that the package was addressed to a psychologist at the university.

"Sources say the letter was from a pent-up Holmes to one of his professors," the news organization reported. "In it, he talked about shooting people and even included crude drawings of a gunman and his victims."

School officials said in a statement that a package discovered at the Facilities Services building on Monday had been delivered to the campus by the U.S. Postal Service that same day and was turned over to authorities within hours of delivery.

"This package prompted the building's evacuation at 12:26 p.m. and employees were allowed to return by 3:06 p.m.," the statement said.

School administrators said that Holmes had taken his preliminary examinations at the school on June 7.

A source familiar with Holmes' academic status said the 24-year-old suspect did "poorly" on the oral exam.

Three days later, Holmes initiated his withdrawal from the program.

"It's very unusual, very unusual for a student to withdraw from our program," Dean Barry Shur told reporters on Monday.