The White House announced that President Joe Biden and first lady Jill Biden will visit Uvalde, Texas, on Sunday.

They will travel to Uvalde "to grieve with the community," the White House said in a statement.

-ABC News' Jon Garcia

Even though the school doors were supposed to be locked during the day, it appears the door the gunman entered through was unlocked, Victor Escalon, the Texas Department of Public Safety's regional director for south Texas, said at a press conference Thursday.

Escalon gave an updated timeline of the shooting:

After crashing his car, the gunman got out through the passenger side with a rifle and bag containing ammunition.

The suspect walked around then saw two witnesses at the funeral home across the street from where he crashed his car and opened fire on them. Neither person was struck.

While in the parking lot, the gunman fired shots at the school multiple times. The suspect then walked into the west side of the building, Escalon said.

The Uvalde Police Department and the Independent School District Police Department attempted to enter the school building four minutes later, but they heard gunfire and took rounds so they moved back, took cover and called for additional resources, Escalon said.

Escalon said the officers "don’t make entry initially because of the gunfire they’re receiving. But we have officers calling for additional resources."

While they made calls to bring in backup, officers evacuated students and teachers.

Approximately an hour later, U.S. Border Patrol tactical teams arrived, made entry and shot and killed the suspect, Escalon said.

Escalon said it then became a rescue operation.

Texas officials said Thursday that police did not confront the gunman before he entered the school, despite earlier reporting that a school district police officer had confronted the gunman.

Police believe the gunman was able to get into the building through an unlocked door at the back of the school, said Victor Escalon, the Texas Department of Public Safety's regional director for south Texas.

The gunman was shot and killed by Border Patrol tactical team nearly an hour after the shooting began, Escalon said.

Uvalde police officers responded to the shooting "within minutes," Uvalde police said in a statement Thursday.

"Our personnel have displayed the upmost commitment to our community during this difficult time as we all are suffering as members of the community, that is the family of Uvalde. It is important for our community to know that our Officers responded within minutes alongside Uvalde CISD Officers," Daniel Rodriguez, Uvalde chief of police, said in a statement.

"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 the questions that we can."

Rodriguez added, "I know words will never ease the pain that we are all suffering, but I hope you will join me in taking some solace in knowing that the pain comes from the fact that we all have such deep love for all the victims who have been taken from us, those who are recovering, and those who only time and love will continue to heal."

Texas state Sen. Roland Gutierrez is counting down the days until the start of the next school year, which he said is 77 days.

Without change, "every day is one day closer to students dying," Gutierrez, a Democrat, told ABC News.

He is among the lawmakers calling on Texas Gov. Greg Abbott to convene a special session to address gun violence.

The senator also mentioned potential plans to tear down Robb Elementary and build a new campus.

"I want what families want. Every person has told me they don’t want to go back. Little kids I’ve met have told me they don’t want to go in that school," he said.

-ABC News' Marcus Moore and Darrell Calhoun