Not only were local police slow to confront the shooter who killed 19 children and two adults in Tuesday's Texas school massacre, they also reportedly refused to allow federal agents to confront the gunman until nearly an hour after they'd arrived on the scene.

Specially equipped US Customs and Border Patrol (CBP) agents, who arrived at the Uvalde, Texas elementary school between noon and 12:10pm, weren't allowed by local police to breach the adjoining classrooms in which the shooter had locked himself until just before 1pm, the New York Times reported on Friday, citing unidentified federal officials.

The federal agents found a chaotic scene when they arrived at the school, where people were pulling children out of windows and police tried to secure a perimeter. The agents, who arrived at the school "far earlier than previously known," didn't understand why they weren't allowed to immediately charge the gunman, the Times said.

"We were told to wait," a Border Patrol official told Yahoo News on Friday. "We were told to wait and wait, and the team wanted to go. But you have to understand, CBP is not the lead agency, so they had to wait, and now look what happened."

Uvalde is located west of San Antonio, about 80 miles from the US-Mexico border. Tuesday's incident began when a 911 caller reported seeing a man with a gun outside Uvalde's Robb Elementary School. The shooter, identified as 18-year-old Salvador Ramos, reportedly entered the school through an unlocked door around 11:40am.