The agents from Border Patrol and Immigration and Customs Enforcement arrived at some point between 12 p.m. and 12:10 p.m., according to the officials — far earlier than previously known. But they did not breach the adjoining classrooms of the school where the gunman had locked himself in until a little before 1 p.m. Members of the federal tactical team killed the gunman.

The officials said that members of the Uvalde Police Department kept the federal agents from going in sooner.

The new details deepened questions about the tactics used to respond to the shooting and the length of time it took officers on the scene to end the carnage.

The director of the Texas Department of Public Safety was expected to hold a news conference about the shooting and the police response on Friday morning.