James Holmes, the accused gunman in last Friday's midnight movie massacre in Colorado, mailed a notebook "full of details about how he was going to kill people" to a University of Colorado psychiatrist before the attack, and the parcel may have sat unopened in a mailroom for up to a week before its discovery Monday, a law enforcement source told FoxNews.com.

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

Among the images shown in the spiral-bound notebook’s pages were gun-wielding stick figures blowing away other stick figures.

The source said police and FBI agents were called to the University of Colorado Anschutz medical campus in Aurora on Monday morning after the psychiatrist, who is also a professor at the school, reported receiving a package believed to be from the suspect. Although that package turned out to be from someone else and harmless, a search of the Campus Services' mailroom turned up another package sent to the psychiatrist with Holmes’ name in the return address, the source told FoxNews.com.

A second law enforcement source said authorities got a warrant from a county judge and took the package away Monday night. When it was opened, its chilling contents were revealed.

The first source on Tuesday told FoxNews.com the package had been in the mailroom since July 12, though another source who confirmed the discovery to FoxNews.com could not say if the package arrived prior to Friday's massacre. It was not clear why it had not been delivered to the psychiatrist.

The notebook is now in possession of the FBI, sources told FoxNews.com.

The University of Colorado Denver issued a statement Wednesday evening confirming that a suspicious package was found, but called the July 12 timeline "inaccurate." The university said it was delivered Monday and found on the same day.

When told of the university's statement, a source said the package may have been postmarked on July 12, but arrived before the massacre.

On Tuesday afternoon, in response to FoxNews.com’s request for comment prior to publication of this article, the same university spokeswoman, Jacque Montgomery, had said only that she was not aware of the contents of the package or who had sent it.

The university also denied Wednesday that the package "sat on a loading dock," though there was no mention of a loading dock in the original FoxNews.com story. The story reported the package was instead found in a mailroom.

Both of FoxNews.com's sources said the intended recipient of Holmes’ notebook was a professor who also treated patients at the psychiatry outpatient facility, located in Building 500, where the first suspicious package was delivered. It could not be verified that the psychiatrist had had previous contact with Holmes, who was a dropout from the school’s neuroscience doctoral program and had studied various mental health issues and ailments as part of his curriculum.

Holmes is accused of killing 12 and injuring 58 at a midnight showing of the Batman movie "The Dark Knight Rises" at the Century 16 Theater in Aurora.

Agent Dave Joly, of the FBI’s Denver Division, declined to comment on the matter, citing a gag order issued Monday by Arapahoe County District Judge William Sylvester. Arapahoe County District Attorney Carol Chambers' office and Aurora police also could not comment due to the gag order.

Police believe the July 20 attack was meticulously planned. Holmes allegedly tossed tear gas canisters into the crowded theater, and then fired his 12-gauge shotgun at the ceiling before turning it on the crowd. As panicked movie watchers raced for the exits, he switched to a .40 Glock pistol and a .223 Smith & Wesson M&P semi-automatic with a high-capacity drum clip, sources told Fox News. The gun jammed, likely preventing far more deaths.

After the gun jammed, Holmes allegedly walked out of the theater through the door he'd entered and was removing his body armor beside his car when he was confronted by the officers who took him down, the source said, adding that the gunman seemed surprised authorities arrived so quickly.

Before mounting the horrific attack, Holmes allegedly booby-trapped his apartment and left music blasting, possibly to create a diversion that would occupy police and rescue personnel several miles away from the theater, the source said.

Fox News has learned that the door was wired with a booby-trap and a backup system that would have triggered an explosive designed to "cut in half" the first person through the door. After that, explosions and flames would have likely consumed the entire building, presumably with the intention of trapping other residents as they slept and forcing a massive response of police and rescue personnel.

Holmes, who made his first court appearance Monday and looked disoriented and disheveled, could face the death penalty.

Editor's Note: This story was updated Wednesday evening to include a statement from the University of Colorado Denver, and a response to that statement from a FoxNews.com source.

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