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.....

But Arredondo was not trying those keys in the door to classrooms 111 and 112, where Ramos was holed up, according to the law enforcement source. Rather, he was trying to locate a master key by using the various keys on doors to other classrooms nearby, the source said.

While Arredondo waited for a tactical team to arrive, children and teachers inside the classrooms called 911 at least seven times with desperate pleas for help. One of the two teachers who died, Eva Mireles, called her husband by cellphone after she was wounded and lay dying.

Days after the massacre, Col. Steven McCraw, director of the Texas Department of Public Safety, said at a news conference that “each door can lock from the inside” and that when Ramos went in, “he locked the door.” That information was preliminary, the source said, and further investigation has yielded new revelations about the door.