This story is part of a KXAN series of reports called “Stop Mass Shootings,” providing context and exploring solutions surrounding gun violence in the wake of the deadly Uvalde school shooting. We want our reports to be a resource for Texans, as well as for lawmakers who are convening a month after the events in Uvalde to discuss how the state should move forward. Explore all “Stop Mass Shootings” stories by clicking here.

UVALDE, Texas (KXAN) — The 18-year-old gunman who killed 19 children and two adults at a Uvalde elementary school was inside the school for approximately an hour before a tactical team with the U.S. Border Patrol killed him, a Texas Department of Public Safety spokesperson said Thursday.

Texas DPS held a media briefing Thursday afternoon in the wake of questions and criticism about law enforcement’s response to the mass shooting at Robb Elementary School earlier this week.

“It takes day, hours, takes time [to investigate],” DPS South Texas Regional Director Victor Escalon said Thursday. “We’re still grabbing a lot of information.”

Despite early reports, Escalon said no officer was on campus before the shooting happened. That contradicted what Gov. Greg Abbott shared at a briefing Wednesday when he said an officer with Uvalde Consolidated Independent School District police approached the 18-year-old shooter as he was heading into the school Tuesday.

However, Escalon said the shooter did not encounter a school officer before entering the building. He said the gunman went into a building on the school’s west side “unobstructed” through a door that he said appeared to be unlocked.

The DPS official provided an updated timeline of the events during the briefing.

At 11:28 a.m. Tuesday, the shooter wrecked the truck that he took from his grandmother outside of the school. The 18-year-old had just previously shot his grandmother, who is alive and stable, the official said.

Witnesses near the school said he was carrying a long rifle and a backpack with ammunition, the DPS spokesperson said.

At 11:40 a.m., the shooter entered the school on the west side of the building after climbing over a fence. DPS said the door appears to have been unlocked.

The shooter fired “numerous rounds” inside the building, DPS said. Officials are still working to figure out how many shots were fired. The “majority” of the shooting happened at the beginning of the incident, DPS said.

Four minutes later, police officers went into the school. The shooter and officers began firing at each other, DPS said.

The shooter continued walking through the building, going into a dual classroom with a door open in the middle, DPS said.

While officers were being shot at, they worked to remove other students and teachers inside of the school, the spokesperson said.

There was a time period of negotiation between officers and the shooter, DPS said. During this time, the tactical team went into the classroom where the gunman barricaded himself and shot and killed him about an hour after he first walked into the school, DPS said.

Uvalde Police Chief Daniel Rodriguez said in a statement two officers were shot by the 18-year-old. Rodriguez said the officers’ injuries are not expected to be life-threatening.

Additionally, Rodriguez’s statement provided some details on his officers’ response to the shooting.

“[Uvalde] officers responded within minutes alongside Uvalde CISD officers,” the statement reads.

“I understand questions are surfacing regarding the details of what occurred. I know answers will not come fast enough during this trying time, but rest assured that with the completion of the full investigation, I will be able to answer all of the questions that we can,” the statement reads.

The Associated Press contributed to this story.