According to statements from law enforcement and other public officials, this is how Tuesday's massacre unfolded.

May 17 - Ramos legally buys a semiautomatic rifle on his 18th birthday at a sporting goods store in Uvalde.

May 18 - He purchases 375 rounds of ammunition from an unknown vendor.

May 19 - The 18-year-old buys a second rifle at the same sporting goods store.

May 24

* 11am to 11.15am - Ramos sends a series of three private social media messages.

"I'm going to shoot my grandmother"

"I shot my grandmother" (she survives the attack with a gunshot wound to the face)

"I'm going to shoot an elementary school".

* 11.28am - Ramos wrecks his truck near the school after fleeing from the house where he shot his grandmother. He jumps out of the passenger side with a rifle and a bag. He sees two witnesses at a funeral home across the street and fires at them before walking towards Robb Elementary School, climbing a fence into the car park and starting shooting.

* 11.40am - The shooter walks into the west side of the school, apparently through an unlocked door, and fires multiple rounds.

* 11.44am - Officers from multiple law enforcement agencies begin to converge on the school. The gunman shoots at the first officers, who move back and take cover, before approaching the suspect again.

Around this time, the shooter enters a classroom and massacres the students and teachers inside. Officers report hearing at least 25 gunshots from the classroom soon after arriving at the scene.

* 11.40am to about 12.40pm - Officers on the scene call for more help, requesting tactical teams, body armour, precision snipers and hostage negotiators.

* About 12.40pm - U.S. Border Patrol tactical teams arrive, enter the classroom, and kill the suspect.

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Australian Associated Press