Diego said his sister called him right away after a suspected shooter, who police identified as a 12-year-old girl, opened fire and shot two 15-year-olds at the Westlake district campus.
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The young girl informed her brother that a shooting had just occurred, and she asked her sibling to come for her.
"I went insane. I didn't know what to do, I didn't know what to say, and that's when I just got my mom and came over here," Diego said.
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He added that his sister hasn't had a chance to speak to him about the incident but said she simply told him, "Someone just got shot, and I need you to be here right now. I'm scared."
Without thinking, Diego told her he will be right there, and everything was going to be OK.
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"As an older brother you wouldn't like your little sister to be in this type of situation, you know," an emotional Diego said.
Diego, while still wearing medical scrubs, said he was in class when he heard from his sister. He said he quickly grabbed his things and left his school so he could get to his sister as fast as he could.