The report cites law enforcement close to the investigation as their source.
That source reportedly told the Express-News that police may have assumed the doors were locked and the shooter could not have locked the doors from the inside. Pete Arredondo, the district police chief in charge of law enforcement's response at Robb, has said previously that he went through a ring of keys provided by a janitor in order to try and gain entry.
The Express-News, however, reports that he was instead trying other doors nearby in an attempt to locate a master key.
The classroom doors at Robb Elementary are designed to lock automatically when they are closed with a key as the only way to open the doors on the outside, the source told the newspaper.
The Robb Elementary School shooting occurred in Uvalde when 19 children and two teachers died after an 18-year-old shot them with an AR-15 style rifle on May 24. The aftermath has been defined by scrutiny of law enforcement by state leaders and local families seeking answers.
Meanwhile, dozens of requests have been made for surveillance footage and other records pertaining to the shooting, and Uvalde has hired legal assistance to try and keep those records private.