The new development in the investigation of the shooting came just days after Chief Pete Arredondo of the Uvalde Consolidated Independent School District Police, the incident commander during the May 24 mass shooting at Robb Elementary School, defended his actions and claimed the delay in breaching connecting classrooms 111 and 112, where the gunman was holed up was because he was waiting for a janitor to get the key to the door.
But surveillance footage showed that neither Arredondo nor any other officers taking cover in the hallway outside the classrooms ever attempted to open the door before receiving the keys to the two connecting classrooms. That means there were 77 minutes between when the alleged 18-year-old gunman entered the school through an unlocked door and when police fatally shot him, a source with knowledge of the investigation told ABC News.
SEE ALSO: Uvalde police who delayed entry into school during shooting were waiting for protective gear, official says
The San Antonio Express News was the first to report on Saturday that Arredondo and his team allegedly never check the classroom doors to determine if they were unlocked.
The sources confirmed to ABC News that investigators now believe the alleged gunman, Salvador Ramos, could not have locked the doors to the classrooms from inside as officials first suspected. In the surveillance footage, the sources said, it appears Ramos, 18, was able to open the door to classroom 111 from the outside, the source said. That classroom is connected to the adjacent classroom 112 by a short corridor where a restroom is located, officials have previously said.
Whether the doors to the classrooms where the slayings occurred were unlocked through the entire episode remains under investigation.
MORE: Teacher who survived Uvalde shooting calls police 'cowards,' says he will 'never forgive them'
In a June 6 interview with ABC News' Amy Robach, Robb Elementary School teacher Arnulfo Reyes, who was wounded in the shooting that killed 11 of his students, said that prior to the rampage he complained to the school's principal that the door to his room, 111, did not latch properly during security checks. He said the door was supposed to remain shut and lock automatically.
"When that would happen, I would tell my principal, 'Hey, I'm going to get in trouble again, they're going to come and tell you that I left my door unlocked, which I didn't,'" Reyes said in the interview. "But the latch was stuck. So, it was just an easy fix."
In an interview with the Texas Tribune published June 9, Arredondo, who was recently sworn in as a Uvalde City Council member, said he spent more than an hour in the school hallway calling for tactical gear, a sniper and keys to get inside the classroom.
He claimed he and multiple officers with him in the hallway took cover away from the classroom doors for 40 minutes to avoid being struck by bullets the suspect, armed with an AR-15 style rifle and hundreds of rounds of ammunition, fired through the door.
WATCH: How law enforcement narrative of Uvalde mass shooting changed
The Texas Department of Public Safety said three Uvalde police officers who initially ran into the school to confront the gunman were fired on through the door and two suffered graze wounds.
Law enforcement officers from multiple agencies in the area converged on the school and began evacuating children from other classrooms and away from the two rooms where the gunman was holed up. Video and photos from the scene, showed children being pulled through broken windows and running out of harm's way.
Arredondo claimed in the interview with the Texas Tribune that a custodian finally brought him a large key ring with dozens of the keys attached but none worked. Sources familiar with the investigation claimed that while searching for a master key, Arredondo tried the janitor's keys on a door out of harm's way on a nearby classroom.
While Arredondo waited for the keys and a tactical team to gear up and reach the scene, students and teachers trapped in the classrooms with the gunman made at least seven desperate 911 calls asking for help.
Arredondo told the Texas Tribune that he didn't bring his radios with him to the scene, claiming time was of the essence and that he wanted to have his hands free.
"The only thing that was important to me at this time was to save as many teachers and children as possible," Arredondo told the Texas Tribune.
SEE ALSO: Experts say proposed federal gun safety measures might not have prevented Uvalde school shooting
Sources told ABC News that Arredondo is not cooperating with investigators probing the shooting. Arredondo has denied he has been uncooperative.
Arredondo and police involved in the response to the deadly emergency have come under intense scrutiny as the investigation has unfolded and video surfaced showing panicked parents being held back by police officers from entering the school to take matters into their own hands, including a father who officers deployed a stun gun on and a mother who was handcuffed.
Police investigators and elected leaders, including Texas Gov. Greg Abbott, have also been the subject of scorn over how the official narrative of the rampage has dramatically changed as the investigation has unfolded.
In the immediate aftermath of the Uvalde rampage, Abbott praised the "amazing courage" of law enforcement, saying the incident "could have been worse" if the officers hadn't run toward the gunfire and eliminated the shooter. But one day later, Victor Escalon, the South Texas regional director of the Texas Department of Public Safety, contradicted Abbott's statement, saying, no schools officer was at the campus when the gunman, who had already shot and wounded his grandmother, crashed a truck in front of the school and entered the school buildings unabated through an unlocked door after getting onto campus by climbing a fence.
Abbott later said he was misled on the police response.
Col. Steven McCraw, director of the Texas Department of Public Safety, initially said the door the gunman used to access the school building was left propped open by a teacher. But officials later said the investigation showed the teacher closed the door, but the door did not automatically lock as it was supposed to.
The timeline on how quickly police responded to the shooting has also changed several times, from a rapid response to about 40 minutes, to eventually 77 minutes before a SWAT team entered the classroom where the shooter was located and killed him, authorities said.
The New York Times reported on Friday that a Uvalde police officer responding to initial reports of a shooting was armed with an AR-15 style rifle and had an opportunity to shoot the gunman outside the school but hesitated out of concern he could have hit a student with an errant shot. Law enforcement sources have confirmed that scenario to ABC News.
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The number of people injured or killed does not include the suspect or perpetrator. These graphics show the number of victims across all mass shootings from the last five years.