A gunman who killed 19 children and two teachers at a Texas elementary school, in what is now the deadliest school shooting in the states history, shot his grandmother and posted about his plans on social media before carrying out his attack, authorities said.

Authorities were called to Robb Elementary School in Uvalde, about 80 miles west of San Antonio, just before 12 p.m. Tuesday. There, they said they encountered the 18-year-old gunman after he crashed his vehicle outside.

The lone gunman, identified as local resident Salvador Ramos, was armed with an AR-15-style semi-automatic rifle and hundreds of rounds of ammunition that he had purchased a week before. A second rifle was found in his abandoned truck, according to a law enforcement briefing obtained by state Sen. John Whitmire (D) of Houston.

Ramos shot his grandmother before driving to the nearby school, which educates students in the second, third and fourth grades, Texas Department of Public Safety Lt. Christopher Olivarez told CBS Mornings. Local police, school officers and state troopers met him there.

They heard the gunfire, they saw the shooter. At that point they were met with gunfire by the shooter, Olivarez said. Several police officers were shot. At that point, the gunman, the shooter, was able to make entry into a classroom, barricaded himself, and at that point just started shooting children and teachers that were inside that classroom, having no regard for human life.

In a separate interview with CNN , Olivarez said all the victims were in the same fourth-grade classroom at Robb Elementary.

Officers at the scene were initially unable to enter the classroom, so their primary focus became evacuating as many children from the school as they could. But again they kept hearing the gunfire as they were doing this, as they were performing the rescues, Olivarez said.

A specialized tactical law enforcement team arrived and was able to make forcible entry into the classroom with the gunman. One of the officers in that team was shot by the gunman upon entry, but at that point they were able to shoot and kill the shooter and prevent any further loss of life, Olivarez said.

The shooter was wearing a tactical vest, but lawmakers were later told that it did not contain any hardened body-armor plates, The Associated Press reported .

The Texas Tribune has reported that the grandmother survived the shooting and was airlifted to an area hospital.