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(CNN) - Sheltering in his classroom as shots rang out, Arapahoe High School student Matt Bowers fumbled for a pen in his pocket, and hastily scrawled a goodbye note to his family on his own hand.
"It said, 'Family, I love you all so much,' and I underlined the 'so much' because I really meant it," Bowers said in an exclusive interview with CNN's "The Lead with Jake Tapper."
Underneath, the young Christian wrote, "I'm here now" and drew a cross, symbolizing he was in heaven.
"That's where I really genuinely thought I was headed if Karl happened to stumble into our classroom and actually end it for all of us," said Bowers.
Officials identified the shooter as 18-year-old Karl Pierson. Pierson shot one student before turning the gun on himself and taking his own life.
During the chaos of the shooting, Bowers says he remembers praying, and feeling around in his pockets to see what he had on him.
"I found that I was carrying a pen with me, a ball point pen, and that morning, I didn't really tell my family I loved them," said Bowers. "I wrote, 'I love you' on my hand just so that they knew I was thinking about them, and I was praying for them."
"Since the shooting, I have been carrying the pen in my pocket the whole time," said Bowers.
The young student told his harrowing story in a blog post published Saturday night, writing, "My entire life had changed in the course of 30 seconds."
"Really it was just a normal day when it all started. The first loud bang we heard from the hallway, we didn't really quite know what it was," Bowers told CNN.
Seconds later, when two more shots rang out, everybody realized what was happening.
"I remember I looked at my teacher's face and it just became completely pale," said Bowers.
Everyone rushed to the corner of the classroom, away from the doors and the window, trying to climb to safety as fast as they could.
"I was climbing over desks to get to the corner," said Bowers.
At the end of that 30 seconds, as a voice came over the intercom alerting everyone to an immediate lock down, Bowers remembers thinking, 'This is really happening, this isn't a joke.'
"I have never had such a frightening experience in my life before like that," said Bowers. "It really changed how I looked at my life, and just my whole perspective on what life is."