The Robb Elementary School gunman had a total of seven 30-round magazines with him in the classroom where the shooting took place, multiple law enforcement sources told ABC News.

One 30-round magazine was in the AR-15-style rifle used in the shooting and six magazines were carried on a tactical vest worn by the shooter, with the potential to hold 210 rounds.

It is unclear how many rounds were expended.

Fifteen additional loaded magazines, potentially holding 450 rounds, were found in a backpack that police said the shooter had when he exited a pickup truck that he crashed near the school.
It is unclear where this backpack was found by police or whether he had it in the classroom.

-ABC News' Jack Date, Luke Barr, Josh Margolin and Pierre Thomas

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.

Those inside a classroom with the shooter made several calls to 911, but the tactical unit that arrived at 12:15 p.m. waited 35 minutes before breaching the classroom, Steven McCraw, director of Texas Department of Public Safety, said at a press conference Friday.

A 911 call was made at 12:03 p.m. from room 112 and lasted 23 seconds. McCraw did not identify the caller.

She called back at 12:10 p.m. and advised that there were multiple dead in the classroom, McCraw said.

The person then called again at 12:13 p.m. and again at 12:16 p.m., when said there were eight to nine students who were still alive, McCraw said.

A call was made by someone else from room 111 at 12:19 p.m., the caller hung up when another student told her to hang up, McCraw said.

At 12:21 p.m., three shots were heard over a 911 call. At 12:36 p.m., another 911 call was made by the initial caller and it lasted for 21 seconds. The student caller was told to stay on the line and be very quiet. She told 911 that the gunman shot the door, McCraw said.

At approximately 12:43 p.m. and 12:47 p.m., she asked 911 to please send the police now, McCraw said.

The caller said she could hear police next door at 12:46 p.m. At 12:50 p.m., the Border Patrol tactical unit finally breached the door and shot the suspect.