The shooter first shot his grandmother, crashed a vehicle near the school and then barricaded himself into classroom as he fired at students and teachers, a spokesperson for the Texas Department for Public Safety has said.
Local law enforcement then received a call about a crashed car and an individual with a weapon making his way into the school, Lt Chris Olivarez told NBC.
"School officers and state troopers were first on scene. They heard gunshots from the classroom and tried to enter, but they were met with gunfire from the shooter. Some of them were shot," he said.
"They began breaking windows, trying to evacuate children and teachers.
"At that point, the shooter made entry into a classroom, barricaded himself into that classroom and began shooting numerous children and teachers, having no regard for human life."
Local officers as well as state troopers were eventually able to force their way into the classroom, he said.
"They were met with gunfire as well but they were but were able to shoot and kill the suspect," he said.
Olivarez said the shooter used a long, platform-type rifle, was equipped with "multiple ammunition" and was wearing a tactical vest - a form of body armour.
He lived with his grandparents, was unemployed and had attended a local school, he added.
