According to Thesmokinggun.com Jeff Weise, 16, sent the animated film under the title Target Practice and the name "regret" in October. Another, titled Clown, depicted a clown strangling a person.
The black and white drawings are spattered with red blood to the sound of gunfire and other explosions. When the shooter kills himself the screen turns red.
William Bastone, editor of the site, left a biography saying Weise's favourite films were the zombie Dawn of the Dead; his favourite recording artists were Korn, Marilyn Manson, Rammstein and John Lennon. He described himself as "nothin' but a Native American teenage-stoner-industrialist".
Law officials are trying to determine the motive for Weise's rampage on Monday.
Some clues are emerging. Weise had been in hospital at least once for suicidal tendencies and was taking an anti-depressant, Prozac, according to a cultural coordinator at Red Lake middle school quoted in the Washington Post.
"I feel like a child in the middle of a dark abyss," Weise wrote in November, according to the Minneapolis Star-Tribune. "I think life is the best soap opera there is ... and the producers are about to bust out their most 'entertaining' plot lines."
The Red Lake reservation remained in lockdown, with media confined to a pen and locals told to eschew interviews. Tribal chiefs said they wanted to avoid a media circus and a national emotional outpouring.
"We are trying to avoid the situation that occurred in Columbine," Floyd Jourdain, 40, chairman of the Red Lake Band of Chippewa Indians, said, referring to the nation's worst school shooting six years ago."We have an immense job ahead of us in trying to heal our community."