The information was attributed to unnamed law enforcement sources.
FoxNews.com said the package, sent to a University of Colorado psychiatrist, went unopened until Monday.
"There were drawings of what he was going to do in it -- drawings and illustrations of the massacre," FoxNews.com reported Wednesday.
The Denver Post said a law enforcement source confirmed Holmes mailed a notebook to a psychiatrist but the source did not know what it contained, the newspaper said.
FoxNews.com said investigators found a package with Holmes' return address on it that had been sent to a professor who also treats patients in the university's outpatient facility. The package had been in the mailroom since July 12.
"Among the images shown in the spiral-bound notebook's pages were gun-wielding stick figures blowing away other stick figures," FoxNews.com said.
Twelve people were killed and 58 others were wounded when Holmes, 24, allegedly opened fire in an Aurora, Colo., theater last week.
Google buys drone maker Titan Aerospace
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend