Texas state police on Tuesday said an exterior door at Robb Elementary School did not lock when it was closed by a teacher shortly before a gunman used it to get inside and kill 19 students and two teachers.

Initially, state police said a teacher had propped the door open shortly before Salvador Ramos, 18, entered the school in Uvalde, Texas, on May 24.

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They have now determined that the teacher, who has not been identified, propped the door open with a rock, but then removed the rock and closed the door when she realized there was a shooter on campus, said Travis Considine, chief communications officer for the Texas Department of Public Safety.

“We did verify she closed the door. The door did not lock. We know that much and now investigators are looking into why it did not lock,” Considine said.

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Investigators confirmed the detail through additional video footage reviewed since Friday’s news conference when authorities first said that the door had been left propped open. Authorities did not state at that time what had been used to prop open the door.

Considine said the teacher initially propped the door open but ran back inside to get her phone and call 911 when Ramos crashed his truck on campus.

Since the shooting, law enforcement and state officials have struggled to present an accurate timeline and details of the event and how police responded, sometimes providing conflicting information or withdrawing some statements hours later. State police have said some accounts were preliminary and may change as more witnesses are interviewed.

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Investigators are also still trying to interview Uvalde Consolidated Independent School District Police Chief Pete Arredondo, who state police have said was the commander of the school shooting scene while it happened. Arredondo has not responded to multiple requests for comment from The Associated Press.

Later Tuesday, the Combined Law Enforcement Association of Texas, which represents police officers, urged its member officers to cooperate with “all government investigations” into the shooting and police response and endorsed a federal probe already announced by the Justice Department.