Surveillance footage shows that police never tried to open a door to two classrooms at Robb Elementary School in Uvalde in the 77 minutes between the time a gunman entered the rooms and massacred 21 people and officers finally breached the door and killed him, according to a law enforcement source close to the investigation.

Investigators believe the 18-year-old gunman who killed 19 children and two teachers at the school on May 24 could not have locked the door to the connected classrooms from the inside, according to the source.

All classroom doors at Robb Elementary are designed to lock automatically when they close and can only be locked or unlocked from the outside with a key, the source said. Police might have assumed the door was locked. Yet the surveillance footage suggests gunman Salvador Ramos, 18, was able to open the door to classroom 111 and enter with assault-style rifle — perhaps because the door malfunctioned, the source said…

Regardless, officers had access the entire time to a “halligan” — a crowbar-like tool that could have opened the door to the classrooms even if it was locked, the source said.