"This is not a suicide note or a diary," Park County Sheriff Fred Wegener said. "However, many times, the letter references suicide. This letter clearly acknowledges his pending death.
"It also apologizes to his family for his actions that will occur."
The letter was intended for all Morrison's family members, reports Rick Sallinger of CBS station KCNC. It was received in the mail Thursday, the day after the shooting.
The 14-page letter from Duane Morrison contains no reference to Platte Canyon High School or any school, nor is there a time reference, authorities said, leaving investigators without any known connection between Morrison and this mountain town 35 miles southwest of Denver.
Morrison molested all six girls and sexually assaulted at least two of them before police commando teams stormed into the classroom. During a gun battle with police, Morrison shot a 16-year-old girl in the head and fired at authorities, then killed himself.
The letter was postmarked Wednesday in nearby Shawnee — the same day of the attack.
"It doesn't tell me a lot of why, but it does tell me that the conclusion of the events of the 27th might have been my worst fears," the sheriff said. "He probably intended to kill or have us shoot him."
Investigators also traced the handgun used in the shooting to Morrison's brother, Wegener said.
The sheriff declined to elaborate on Morrison's sexual attacks on the students.
One of the hostages, Lynna Long, told the Rocky Mountain News that she was groped above the waist but believes Emily "got it worse." She said she was afraid to look, "But you could hear Emily saying, 'No. Please don't."'
The newspaper said Lynna and her mother had agreed to allow Lynna to be identified by name. The family told The Associated Press that the girl was not immediately available Friday.
Lynna said all the girls had been told to stand facing a wall and she could not see what Morrison was doing, but she knew the other girls were being molested because "You could hear the rustling of clothes and elastic being snapped and zippers being opened and closed."
Authorities say they knew of no connection between Morrison and the hostages he held for four hours after bursting into a college prep English class at the high school.
Wegener said Thursday the suspect approached a male high school student Wednesday and "asked about the identity of a list of female students." The sheriff said he was not sure if it was a written list or names rattled off by Morrison.
It was not disclosed whether the list included Emily Keyes, the girl who was killed.
KCNC-TV reported that video from cameras outside the school showed Morrison sitting in his Jeep in the school parking lot for about 20 minutes and then mingling with students as classes changed, nearly 35 minutes before the siege began.
Video from cameras outside the school showed Morrison sitting in his Jeep in the school parking lot for about 20 minutes and then mingling with students as classes changed, nearly 35 minutes before the siege began, KCNC-TV in Denver reported.
Investigators said Morrison, 53, was a petty criminal who had a Denver address but apparently had been living in his battered yellow Jeep.
Morrison walked inside the school with two handguns and a backpack that he claimed contained a bomb. Investigators did not say what was in the backpack.
During the siege, Morrison released four hostages. While still holding two girls, he cut off contact with deputies and warned that "something would happen at 4 o'clock," authorities said.
About a half-hour before the deadline, police commandos team used explosives to blow a hole in a classroom wall in hopes of getting a clear shot at him. When they could not see him through the gap, they blew the door off the hinges to get inside, said Lance Clem, a spokesman for the state Department of Public Safety.
Morrison fired at the SWAT officers, shot Keyes as she tried to run away and then killed himself, authorities said. During the gun battle, police shot Morrison several times, they said.
Classes were canceled for the rest of the week as the community tried to come to grips with the bloodshed, which evoked memories of the 1999 shooting rampage that left 15 dead at Columbine High School, less than an hour's drive away.
Student Chelsea Wilson said she was in the classroom when the gunman came in and told the students to line up facing the chalkboard.
"All the hairs on my body stood up," she said. "I guess I was somewhat praying it was a drill."
One by one, the gunman started letting students go. Chelsea, a tall brunette, said she was the first to leave. Her mother, Julia Wilson, said she thinks the gunman selected the blond, smaller girls. Keyes' yearbook photo shows a smiling blond girl with blue eyes.