(NewsNation) — Emotions ran high and were palpable as the investigation unfolded after an 18-year-old gunman opened fire on Robb Elementary School, killing 19 children and two adults this week.

“It is so hard,” Victor Escalon, a regional director at the Texas Department of Public Safety, said Thursday during a press conference as he provided a summary of what happened leading up to the mass school shooting in Uvalde, Texas. “We are all hurting inside.”

Escalon said the gunman did not initially encounter any law enforcement officers as he entered the school, debunking earlier reports that he was confronted by an armed school resource officer.

“He walked in unrestricted initially,” Escalon said. “So from the grandmother’s house to the ditch to the school, into the school, he was not confronted by anybody to clear the record on that.”

According to a timeline that Escalon said police are constructing, the gunman crashed a black pickup truck in a ditch and jumped out of the passenger-side door at 11:28 a.m. on Tuesday.

Witnesses reported seeing the gunman holding a long-arm rifle and what turned out to be a bag of ammunition.

The gunman then fired shots at two witnesses who saw the crash from their location at a nearby funeral home, according to Escalon. The gunman also fired numerous shots at the school.

Escalon said the first 911 calls were received at 11:30 a.m.

“We can confirm 11:30 a.m.,” Escalon said. “We got a crash and a man with a gun. And then you have responding officers. That’s what it is.”

The gunman then entered the building at 11:40 a.m. unobstructed through a door that was apparently unlocked, according to Escalon.

When asked why the school door was unlocked, Escalon said, “We will find out as much as we can, why it was unlocked, or maybe it was locked. But right now it appears to have been unlocked.”

Escalon said law enforcement made entry into the school four minutes later.

“Four minutes later, local police departments, Uvalde Police Department, the Independent School District Police Department are inside. Make an entry,” he said. “They hear gunfire, they take rounds. They move back, get cover. And during that time they approached where the suspect is at.”

Investigators say it was about “an hour” later that Border Patrol tactical teams arrived and made entry into the classroom.

Authorities faced mounting questions and anger over the amount of time that elapsed before they stormed the school and put a stop to the rampage.

Frustrated onlookers urged law enforcement to charge into the Texas elementary school, a witness told the Associated Press.

Escalon said the initial responding officers received gunfire and called for additional resources.

“They don’t make entry initially because of the gunfire they’re receiving,” he said.

Juan Carranza, 24, told the AP he saw the scene from the outside of his house across the street from the school. He said he heard women yelling, “Go in there! Go in there!” at officers, but noticed they did not enter the school.

“Initially, there were several officers, local officers, here from Uvalde trying to make entry prior to the tactical team arriving,” Lt. Chris Olivarez explained. “They were met with gunfire. Two officers were shot. They were unable to make entry into the school, and that’s when the gunman barricaded himself inside the classroom.”

During the minutes between the initial 911 call and the tactical team breaching the classroom, law enforcement was evacuating students and teachers, according to Escalon, saying it was a “complex situation.”

Escalon said, “The majority of the gunfire was in the beginning.”

“Multiple rounds, numerous rounds are discharged in the school,” he said. “I say numerous, more than 25. I mean, it was a lot of gunfire in the beginning.”

Once the classroom was breached, it turned into a rescue operation, he said.

In addition to the 19 children and two adults who were killed, 17 others received injuries called non-life-threatening in what is now the deadliest school shooting in Texas history.

The assault is the deadliest shooting at a U.S. school since a gunman killed 20 children and six adults at Sandy Hook Elementary in Newtown, Connecticut, in December 2012.

A Robb Elementary Memorial Fund account has been opened at First State Bank of Uvalde for the families of Robb Elementary, according to the school district. Donations will be accepted at all FSB branches. Checks should be payable to the “Robb School Memorial Fund” and can be mailed to 200 E. Nopal St. Uvalde, TX 78801. Donations through Zelle can be sent to robbschoolmemorialfund@gmail.com, according to the school district.