Chief Arredondo did not have a police radio with him, according to a law enforcement official familiar with the investigation, which may have impeded his immediate ability to communicate with police dispatchers. As two supervisors from the local police department were grazed by bullets fired by the gunman, he made a decision to fall back, the official said.

Using a cellphone, the chief called a police land line with a message that set the stage for what would prove to be a disastrous delay in interrupting the attack: The gunman has an AR-15, he told them, but he is contained; we need more firepower and we need the building surrounded.

Rather than confront an actively shooting gunman immediately, as officers have been trained to do since the killings at Columbine High School in 1999, the ever-growing force of increasingly armed officers arriving at Robb Elementary held back for more than an hour.