(WKBN) — The U.S. Department of Justice will now review the actions taken by law enforcement in the minutes after an 18-year-old walked into a Texas elementary school and started shooting.

Officials with the Texas Department of Public Safety say nearly an hour passed until a U.S. Border Patrol tactical team breached the door to a classroom where the gunman was hiding and killed him.

Mahoning County Sheriff Jerry Greene said tactics used by police in active-shooter situations have evolved over the years.

“You know, at first, twenty, twenty-plus years ago it was to…get a command post or stage here. It’s changed from that now. The goal now is as long as there’s active shooting is to go in and eliminate the threat,” said Greene.

Authorities say in that time before the gunman was shot 19 young children and two of their teachers were killed.