Michigan State professor Marco Diaz-Munoz has been teaching at MSU since 2008. He was in his classroom when the first shots rang out Monday night. "The first thing that comes to my mind is this cannot be happening," he says. "Because how can you believe that this is happening to you." Professor Diaz-Munoz was teaching in Berkey Hall at around 8:15 p.m. Monday just like he has countless times. He says he never expected a gunman to enter through the classroom's back door and begin firing at students. "It’s horror and incredible pain," he said. "They are the age of my daughters so it’s like seeing my daughter going through that so it was so painful to see someone doing that harm to them and me being unable to stop it."