Authorities in Texas are working to "gather the facts" to establish a concrete timeline for the deadly shooting at Robb Elementary School in Uvalde, Texas Department of Public Safety spokesperson Lt. Chris Olivarez told CNN on Thursday.
Here's what we know and what questions remain about the massacre:
School resource officer was armed
Olivarez told CNN's John Berman that officials are "trying to establish and corroborate exactly what was that role" for the school resource officer when he encountered the gunman at the school.
Texas Rangers conducted an interview last night with the school resource officer, Olivarez said. The officer was armed at the time of the shooting, he said, but it's unclear if he fired his weapon.
Yesterday, Texas Department of Public Safety Sgt. Erick Estrada said that the suspect dropped a bag full of ammunition outside of the school prior to entering.
School door was unlocked
The gunman was able to enter the school "unimpeded" by any locks, Olivarez said, and gunfire was exchanged inside the school hallway between the gunman and officers who were right behind him.
Those two officers were shot, and then the gunman barricaded himself inside a classroom, Olivarez said. Officials are still trying to determine exactly how he barricaded himself inside.
CNN previously reported that the gunman was on the school premises for up to an hour before law enforcement forcibly entered a classroom and killed him.
When asked why it took tactical teams so long to respond to the shooting, Olivarez said that they are working to establish an accurate timeline, which is part of the investigation.
"Right now, we do not have an accurate or a concrete timeline to provide to say 'the gunman was in the school for this period,' so we want want to obtain that factual information once we're able to attain that," the lieutenant said.
Parents seek answers
The father of a victim of the shooting told The Washington Post that he and other dads wanted to storm the elementary school to retrieve their children as they heard gunshots from inside. Video posted to social media appears to show frustrated and distraught parents and other adults outside the school clashing with law enforcement officers, urging the officers to go inside and get the gunman or let them go inside themselves.
"I can tell you right now, as a father myself, I would want to go in too. But it's a volatile situation. We have an active shooter situation. We're trying to preserve any further loss of life. ... We cannot have individuals going into that school, especially if they're not armed," Olivarez said.
Olivarez also said local authorities are working with the FBI to obtain surveillance video from the school.
The FBI is also doing cell phone forensics for the gunman to "establish or obtain information from the suspect's phone, if there was any social media posts that was posted, any other facts that would help the case in terms of as far as there were any indicators for this shooter leading up to this mass shooting," he said.