Sheriff Fred Wegener did not release the text of the letter but said investigators were analyzing it and may make more details public later in the day.
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, and is about 10 pages long and rambling.
In the letter to his family, Morrison says he is going to take his life. There will be publicity, and they will be angry. But he does not say he is going to go into a high school to carry out his death wish, reports Sallinger.
The brother lives in Denver.
The sheriff said investigators learned of the letter Thursday when federal agents traced the handgun Morrison used in the attack to a relative, who then told investigators he had received the note. Wegener did not identify the relative or say when the note was written or delivered.
Authorities say Morrison shot and killed 16-year-old Emily Keyes on Wednesday before killing himself as SWAT team members swarmed into the Platte Canyon High School classroom in a rescue attempt.
Morrison molested all of the six girls and sexually assaulted at least two of them, Wegener said Friday. He declined to elaborate.
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.
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.
On Thursday, Wegener said the suspect approached a male high school student Wednesday and "asked about the identity of a list of female students." The sheriff said he wasn't sure if it was a written list or names rattled off by Morrison.
It was not disclosed whether the list included Emily Keyes.
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.