The police officers who waited more than an hour inside a school in which a mass shooter was barricaded were in a position that received only sporadic and unreliable radio communications, according to two officials with knowledge of the investigation and the system.

The lack of direct and clear communication complicated the response to the mass shooting that left 19 fourth-graders and two teachers dead, said a U.S. official with knowledge of the investigation. For over an hour, law enforcement waited in a hallway, instructed not to rush into a classroom in which the gunman had barricaded himself, while children inside were calling 911 pleading for help.

Ultimately, 19 children and two teachers were shot to death in the classroom on May 24 before federal agents from a Border Patrol tactical team known as Bortac breached the doors and killed the shooter.