The University of Colorado Denver said Wednesday that the U.S. Postal Service delivered the package Monday, and it was immediately investigated and turned over to authorities within hours.
It wouldn't confirm its contents or whether it was sent by former neuroscience graduate student James Holmes. However multiple media outlets, citing unnamed sources, reported Holmes sent a notebook with drawings and descriptions of an attack.
Fox News' website was among those reporting the notebook was in a package addressed to a psychiatrist at the school. It was unclear if Holmes, 24, had had any previous contact with the person. The neuroscience program that he withdrew from on June 10 included professors of psychiatry.
Holmes is accused of opening fire on a theater showing the new Batman movie, killing 12 people and injuring 58. He is due to hear the charges against him at a court hearing scheduled Monday.
NBC News, citing unnamed sources, reported that Holmes told investigators to look for the package and that it described killing people.
The FBI and other law enforcement agencies refused to confirm the reports to The Associated Press.
U.S. Postal Service spokesman David Rupert said the agency's inspectors have no direct knowledge of the package in question. He said no one has contacted the Postal Service for assistance in the investigation.
Before a judge issued a gag order in the case, police said Holmes received more than 50 packages at the school and his home that apparently contained ammunition, combat gear and explosive materials that he used in the attack and to booby-trap his Aurora apartment.
The apartment building was evacuated for days while authorities rendered it safe and collected evidence. Residents were allowed to return Wednesday.
Holmes' defense team also briefly visited the building Wednesday and left without answering reporters' questions.
Holmes, who grew up in California, was allegedly stockpiling for the attack while he studied at the school's neuroscience program. He bought a shotgun and pistol in May, authorities say. On June 7, the date he took a year-end oral exam, he bought an assault rifle. He filed paperwork to leave the program three days later and did not provide a reason, the university has said.
Meanwhile, 51-year-old Gordon Cowden was honored Wednesday at the first memorial service for a slain victim.
About 150 mourners including Colorado Gov. John Hickenlooper gathered on Wednesday the father who had gone to see "The Dark Knight Rises" with his teenage children. His children were unharmed.
Cowden lived in Aurora, the Denver suburb where the theater is located. A family statement described him as a "true Texas gentleman" who loved the outdoors and owned his own business.
"A quick-witted world traveler with a keen sense of humor, he will be remembered for his devotion to his children and for always trying his best to do the right thing, no matter the obstacle," his family said.
Cowden's memorial was also attended by Aurora Mayor Steve Hogan and Aurora Police Chief Dan Oates.
Later this week, families of other victims planned to say their final goodbyes.
Funerals were planned in towns from San Antonio, home of aspiring sportscaster Jessica Ghawi, to Crystal Lake, Ill., hometown of Navy intelligence officer John Thomas Larimer.
Pierce O'Farrill, who survived being shot three times in the attack, told The Denver Post he has forgiven the gunman and feels sorrow for him.
"I want to see him sometime. The first thing I want to say to him is `I forgive you,' and the next is, `Can I pray for you?"' he told the newspaper.