Authorities in Texas, United States, said on Friday that police officers waited for nearly an hour outside the school classroom where a gunman shot dead 19 children and two adults on May 24, The Associated Press reported.

“From the benefit of hindsight... it was the wrong decision, period,” Texas Department of Public Safety Director Steven McCraw said at a press conference. “From what we know, we believe there should have been an entry as soon as you can.”

The attack at the Robb Elementary School in Uvalde city was carried out by 18-year-old Salvador Ramos, who was eventually killed by responding officers. Ramos had shot his grandmother before attacking the school.

He had also posted about his plans on Facebook.

Texas Governor Greg Abbott had said that Ramos’ grandmother survived and called the police. He had also said that Ramos had no known history of crime or mental health problems.

Tuesday’s attack at the school was one of the deadliest shootings at an American school since a gunman killed 26 citizens, including 20 children, at Sandy Hook Elementary School in Connecticut in 2012.

The incident has also led to heavy criticism of the Texas Police for their decision to wait for a special tactic unit instead of confronting the shooter.

At Friday’s conference, McCraw said that the on-scene police commander believed Ramos was alone in the classroom and no survivors were left after his initial shooting spree.

“I’m not defending anything, but you go back in the timeline, there was a barrage, hundreds of rounds were pumped in four minutes into those two classrooms,” McCraw said. “So the belief is that there may not be anybody living anymore and that the subject is now trying to keep law enforcement at bay or entice them to come into suicide by contact.”

However, he also said that a phone call made to the 911 – the emergency telephone number in the United States – from inside the classroom had reported eight or nine children still alive, according to The New York Times.