Police recovered it on Monday after getting a warrant for the medical center mailroom to search for the package and then getting a second warrant to actually open it.
When police went to the mailroom, they found several suspicious packages with no return address thinking those might be from Holmes, so they opened those packages first. But it turned out they had nothing to do with the case.
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Then, they came across a package from Holmes as he described it, including his name in the return address. It is now being analyzed.
NBC News' sources will only say that the package contained a notebook with writings about killing people but couldn't go into any more details.
Officials at the University of Colorado Anschutz Medical Campus tell 9Wants to Know the package was delivered to the campus by the United States Postal Service on Monday and was immediately investigated before being turned over to the FBI within hours of the initial delivery.
The Facilities Services was evacuated at 12:26 p.m. due to the package and employees were allowed back into the building by 3:06 p.m.
Anonymous law enforcement sources leaked information about that notebook and its contents to the national media.
Arapahoe County District Judge William Sylvester issued a gag order on Monday that is supposed to prevent sensitive information from being released to the media.
Judge Sylvester also banned cameras and electronic equipment from the courtroom when Holmes is formally charged on Monday.
Criminal defense attorney and former public defender David Kaplan says leaks like this could hurt the prosecution.
"The public has a right to know, and the media has a right to investigate and explore facts, but there are certain things that come to the attention of the prosecution that the determination of how it is to be introduced should be made in the court of law," Kaplan said. "We are all under ethical obligations as officers of the court and parties, not to try the case in the public domain."
Dispatch recordings show Aurora police were inside the theater less than 3 minutes after the first 911 calls at 12:39 a.m.
It took another 20 minutes for medical personnel to get close enough to treat the most badly injured victims, those inside or just outside the theater.
In dispatch recordings, police call repeatedly for medical backup from 12:42 a.m. to 12:59 a.m.
Some wondered if the delay was the result of Aurora's scene safe protocol, where police must declare a scene safe before rescuers can move in.
In a statement to 9Wants to Know, Aurora Fire Captain Allen Robnett said:
"Scene safety created no delay in the treatment and transport of victims on July 20. The first ambulance arrived at the scene within 3 minutes and the first two fire units within 5 minutes after the first call to fire dispatch. Personnel from both immediately began treating patients. All patients that were transported by Aurora were enroute to area hospitals within 55 minutes.
It's natural for people who weren't there and didn't experience what the first responders experienced to question what happened and how quickly it happened. We'll be doing that too. We will evaluate every aspect of the response and learn from it, as we do with every incident.
In mass casualty situations, especially ones of this magnitude, there is inherent chaos that can be difficult to mitigate. All of our first responders-police, fire and medical response personnel-worked as quickly as possible to diffuse the situation and evaluate, treat and transport victims to area hospitals by any means possible.
It is just not realistic for everything to happen instantly. It was an intense situation that challenged our resources. In spite of that, we believe our personnel responded admirably.
We are proud of our responders and are confident everyone did their very best."