The student who shot dead at least 30 people at Virginia Tech sent a package to the US TV network NBC News on the day of the shootings, police said.
Cho's package to NBC contained pictures of himself with guns
The package contained "disturbing" photographs, video and writings, NBC said, posted from the college campus between the two rounds of killings.
On Thursday, police lamented the NBC decision to show the footage.
A total of 33 people, including the gunman himself, died in shootings at two locations on Monday.
In the videos Cho Seung-hui is shown pointing guns at the camera, and ranting angrily.
"You had a hundred billion chances and ways to have avoided today," the 23-year-old gunman said angrily, in an excerpt shown on NBC Nightly News.
"But you decided to spill my blood. You forced me into a corner and gave me only one option. The decision was yours. Now you have blood on your hands that will never wash off," Cho said.
"I didn't have to do this. I could have left. I could have fled. But now I will no longer run," the gunman said.
Police said on Thursday that they were disappointed at NBC's decision to broadcast some of Cho's footage.
But Steve Flaherty, superintendent of Virginia State Police, denied he was worried the videos could encourage others.
"A lot of folks saw images that were very disturbing," he told a news conference.
"This is a kind of image that people in my line of work have to see, and I'm worried that people who are not used to seeing them had to see them."
Police have already revealed that Cho was admitted to a mental health unit in late 2005.
He was sent for evaluation after two female students made complaints against him, they said.
NBC said a time stamp on the package indicated it was sent in the two-hour interval between two killings in a dormitory and a second, massive round of killings in a classroom building.
The envelope, sent to NBC's headquarters in New York, was slightly misaddressed and carried the wrong postal zip code, which delayed its delivery by a day.
The TV network said the package was received in Wednesday morning's mail.
It contained an 1,800-word diatribe and 43 photos, 11 of them showing Cho aiming handguns at the camera. He also sent 28 video clips.
Head of NBC News Steve Capus said it had a "rambling, manifesto-like statement embedded with a series of photographs."
Mr Capus said the material was "hard-to-follow... disturbing, very disturbing - very angry, profanity-laced," but did not contain any images of Monday's shootings.
Two people were killed at the West Ambler Johnston Hall, a university dormitory, at 0715 on Monday.
Two hours later Cho killed 30 others, plus himself, at the Norris Hall complex across campus.
Authorities have not yet linked the 23-year-old to any of those he killed.
Police say the same gun was used at both locations but have not definitely proved that Cho was at West Ambler Johnston at the time of the shootings there.
The complaints by students against Cho were made in November and December 2005, around the time Cho's English teachers raised concerns over his writing and general behaviour.
In the aftermath of the shootings, teachers and fellow students have spoken of Cho's extreme moods, violent writings and unpredictable behaviour.