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The video is difficult to watch, as the children are clearly in pain and crying out in distress.

In Pics | Deadly shooting at Texas elementary school

A teenage gunman murdered at least 19 children and two teachers after storming into a Texas elementary school on May 24. The carnage began with the 18-year-old suspect, identified as Salvador Ramos, shooting his own grandmother, who survived, authorities said. This is the latest bout of gun-fuelled mass killings in the United States and the nation's worst school shooting in nearly a decade

Teenage gunman kills 18 children at Texas elementary school

Investigators believe Ramos shot and killed his grandmother before going to the school, CBS News reported, citing unidentified law enforcement sources