(CBS News) - No one saw the movie theater massacre coming, but a package may provide some insight into the moments leading up to the shooting.
Sources tell CBS News suspect James Holmes mailed a package to the University of Colorado-Denver/Anschutz Medical Campus where he had been a student.
The mail was sent to a psychologist at the university, but the letter wasn't discovered until Monday.
This was not unexpected. U.S. postal inspectors had been searching through the mailboxes near the suspect's home looking for letters and packages Holmes might have sent out. They didn't find any, but that's because the package had already been sent before the shooting.
On Monday afternoon, investigators scoured the mailroom at the university and found what they'd been searching for: a piece of mail from the suspect in the Aurora, Colo. shooting that killed 12 people and injured 58 at a midnight screening of the new Batman film last week.
Before opening it, the sheriff's bomb squad handled it with a robot and took an x-ray, just in case there were explosives inside.
Sources say the letter was from a pent-up Holmes to one of his professors. In it, he talked about shooting people and even included crude drawings of a gunman and his victims.
Former FBI senior profiler Mary Ellen O'Toole said we may well see more letters from the alleged shooter.
"Particularly because of the complexity of this case and how much this person put into these two different crime scenes," O'Toole said. "They appear to want in my opinion a lot of credit for all their hard work."
The biggest question remains motive. Will the letter Holmes sent give any insight into that? Other cases, like the rambling video sent to a TV network by the Virginia Tech shooter have shown us that even when the suspects try to explain their actions, it rarely makes sense to anyone else.
"I can guarantee you, we will find it unsatisfying because there's nothing that this individual could say to us, the general public, to sit back and say now I understand why you did this," O'Toole said.
This was not random. Police had an investigative lead that put them in that mailroom and apparently the lead came indirectly from Holmes himself. Apparently sometime after his arrest he mentioned that a letter had gone to the school from him, and that's what they were looking for.
Ed. note: An original version of this story stated that the package sent to the university sat unopened for days. In a statement released late Wednesday, the University of Colorado-Denver/Anschutz Medical Campus denied those reports and said the package wasn't delivered by U.S. Postal Service until Monday, July 23:
"Officials at the University of Colorado Anschutz Medical Campus can confirm that 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. This package prompted the building's evacuation at 12:26 p.m. and employees were allowed to return by 3:06 p.m."