Story Highlights• Slain hostage Emily Keyes, 16, used as human shield, shot in head, official says
• Gunman's father tells TV son was "just different" and "a loner"
• Police say suspect Duane Morrison, 53, sexually assaulted his hostages
• SWAT team stormed classroom to end standoff in Colorado high school
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BAILEY, Colorado (CNN) -- As 16-year-old Emily Keyes was being held hostage by a gunman in the last hours of her life, she tapped out one last text message to her family: "I love u guys."
Police released the text message and a photo of Keyes on Thursday, as they continued to investigate the Wednesday incident in classroom 206 at Platte Canyon High School.
Keyes was one of two girls whose screams spurred SWAT teams to storm a Colorado classroom where a gunman was holding them hostage, officials said Thursday.
Duane Morrison, 53, had already sexually assaulted and "traumatized" the students when the raid was ordered to end the Wednesday standoff, Park County Sheriff Fred Wegener said.
Morrison used Keyes as a human shield when the officers burst in. Morrison shot her in the back of her head when she tried to escape, Joe Morales, executive director of Colorado's Department of Public Safety, told reporters.
The gunman then shot himself. When his body was removed from the classroom, authorities found a semi-automatic pistol and a revolver on him, the sheriff said. (Watch sheriff reveal what happened after the gunman was pinned down in a classroom -- 3:05)
Morrison had told the police he had a bomb in his backpack, Wegener said, but after it was over, he said, "It looks like there was nothing in the backpack."
Keyes was taken by helicopter to a Denver hospital where she was pronounced dead.
He released four students one-by-one in a three-hour standoff but negotiations then broke down.
Wegener said the gunman threatened that something would happen at 4 p.m.
"It was then decided that a tactical solution needed to be done in an effort to save the two hostages that were in the room," Wegener said.
The screams from the hostages "moved up the tempo of the operation a bit," Morales said.
Wegener said: "We had to go try and save them."
He added that any motive for the attack was "still a mystery."
In an interview with KOKI in Tulsa, Oklahoma, the gunman's father, Bob Morrison, described his son as "just different" and "a loner."
Mom: My son lied
A purported witness to the standoff said the gunman wanted to take only girls hostage, but his mother said later that her son was lying.
Cassidy Grigg, 16, told NBC's "Today" that the gunman fired a warning shot as he entered the classroom, then lined up the students in front of a chalkboard.
However, Grigg's mother said he was in a nearby computer lab when he heard the commotion. Grigg went to check it out but was turned away by his teacher, his mother said.
Larina Grigg elaborated to the Rocky Mountain News, saying her son "wants to say he's sorry. I know and he knows he made a huge mistake. He lost one of his dear friends."
She added that Cassidy wanted to be a hero and "got caught up in the chaos" of television crews scrambling to interview him.
Grigg told "Today" that Keyes "was one of my first friends." (Read how Emily will be remembered as a 'great kid')
"She was always sweet, she always welcomed people," he said. "She was just friendly, she was a good person in general."
'Scared to death'
Authorities have searched Morrison's car, in which he appeared to be living, the sheriff said.
Wegener said the suspect did not have much of a criminal history and only minor things were on his record.
Bailey is not far from Columbine, where two students killed 12 classmates and a teacher before killing themselves in 1999.
"I know we talk about the Columbine connection," Wegener said. "This is something that has changed my school, changed my community.
"My small county is gone," he added.
"I've gone from upset to angry that this man has done this to our community, has done this to our children," Wegener said.
On Wednesday, Wegener said he was "scared to death" as he was deciding whether to order police to storm the classroom.
"Nobody wants anything to happen like this at their school," said Wegener. He said his own son was in the building. Asked whether he was second-guessing his decision, Wegener said, "I have to go and eventually I have to face a family about the fact their daughter is dead. So what would you do?"