As gunshot victims began to arrive at Littleton Adventist Hospital after a horrifying shooting Tuesday at a local charter school, doctors and nurses were ready. They’d trained for situations like this, but for some staff members it was their third time receiving children wounded in a school shooting.

Located in the suburbs south of Denver, the hospital treated victims of the Columbine High School shooting in 1999. In 2013, it treated the victim of a shooting at Arapahoe High School. The 17-year-old girl died after eight days.

The hospital trains twice a year for a mass casualty incident, which could mean something like a chemical exposure, multiple shootings, or fire, Dr. Mark Elliott told. Elliott, an emergency room doctor for 30 years, added that though the procedures for a school shooting are the same as other mass casualties, it’s tough for staff.

“It changes things incredibly just from an emotional standpoint,” he said. “There’s that feeling of why again, why do we have to go through this again.”

Elliott treated patients after Columbine, and 20 years later, he still wears a pin on his lab coat in memory of the victims. Other staff at the hospital have also worked multiple school shootings, sometimes at the schools their own children attended. For anyone who has kids, the school shootings have been especially hard, he said.

“You tend to put the emotions down until it’s over,” Elliott said.