An Uvalde, Texas, police officer armed with a rifle spotted the gunman outside Robb Elementary School before he entered the building and asked his supervisor for permission to shoot, but the superintendent either did not hear the request or did not respond in time. , allowing the suspect to enter the school, according to a report released Wednesday by the Advanced Law Enforcement Rapid Response Training Center (ALERRT). The gunman killed 19 children and two teachers before a Border Patrol squad finally broke into the classroom and took him out 70 minutes later, a delay that has been sharply criticized by lawmakers, state law enforcement officials and the Uvalde community. . It is unclear why the original Uvalde police officer did not immediately shoot the gunman, who had already begun shooting into classrooms as he walked along the perimeter of the school. “In this case, the UPD officer would have heard gunshots and/or reports of shots fired and observed a person approaching the school building armed with a rifle. A reasonable officer would have concluded in this case, under the totality of the circumstances, that the use was a justified use of deadly force,” the ALERRT center, which is based at Texas State University and provides active shooter response training, wrote in the report. The officer told investigators he was concerned about children being endangered if he lost, but ALERRT noted that the Texas Penal Code says “a person is justified in using deadly force when the person reasonably believes that the deadly force is immediately necessary to prevent the execution of the murder.” UVALDE SHOOTING: OFFICERS WITH RIFLES AND BALLISTIC SHIELD WERE INSIDE SCHOOL FOR 58 MINUTES The officer was about 148 yards from the exterior door the gunman entered, which is well within range of an AR-15 platform rifle, although Texas patrol rifle qualifications do not require officers to fire at targets farther than 100 yards. according to ALERRT. The outside west entrance door was entered by the suspect at Robb Elementary in Uvalde, Texas. (Texas DPS via Twitter) Two other key mistakes were also made that allowed the gunman to enter the building. A different Uvalde school district police officer led the gunman into the parking lot at high speed and did not see him before the suspect entered the school at 11:33 a.m. Third, a teacher closed the outside door minutes before the gunman entered, but did not check to see if the door was locked. Furthermore, even if he checked the door, he did not have the proper key at the time to lock it. UVALDE SHOOTING: 4 FAMILIES SUING GUN, LOOK FOR ANSWERS ABOUT GUNS “If any of these three key issues had worked differently, the tragedy that followed could have been stopped,” ALERRT wrote. A banner hangs at a memorial outside Robb Elementary School, the site of a mass shooting in May that killed 19 students and two teachers, on Friday, June 3. (AP/Eric Gay) The active shooter training center went on to note three key issues when the gunman was found inside the school, including that the classroom door lock was broken. Two teams of officers were positioned at either end of the corridor, creating the possibility that an officer could be hit by friendly fire. and police lost “momentum” when the suspect fired at the first officers who approached the classroom door. CLICK HERE TO GET THE FOX NEWS APP “There is a possibility that police officers will be shot, injured or even killed while responding. This is something every officer should be very aware of when they become a law enforcement officer,” ALERRT wrote. Paul Best is a reporter for Fox News Digital. Story tips can be sent to [email protected] and on Twitter: @KincaidBest.