A 15-year-old private school student in Monterrey, Mexico opened fire on his fellow students and a teacher before turning the gun on himself.
A surveillance-camera video of the shooting that was leaked and posted on social media sites showed the boy squeezing off shots in quick succession with apparently good aim.
Reports added that the gunman, believed to be a student, was taken to hospital in serious condition.
He next shoots the teacher, who was looking in another direction, and she falls to the ground. He walked back to where he had been sitting, reloaded, and shot himself in the chin. One shot the boy missed is the first shot to his own head.
Students who had been cowering beneath desks and chairs then fled the classroom, stepping over the shooter to reach the door.
The spokesman for the state's Security Coordination Group said the attacker had been undergoing treatment for depression and "had psychological problems".
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The spokesman attributed it to "the situation that is happening everywhere".
"The children have access to the internet", Fasci said. This has happened in other countries.
The school accepts students from preschool to the secondary level, according to its website.
"There was a reason why book bags were checked", Fasci said. It was unclear how he got the.22 caliber pistol into the school. A previous police programme had seen students' bags checked at the building's entrances. "I think we're going to have to do it again".
Although school shootings in Mexico are rare, seven years ago, Nuevo León launched a public school security program that included random checks of student backpacks.
At the height of Mexico's drug war between 2008 and 2011, schools in northern Mexico were more concerned about the possibility that stray bullets from drug gang gun battles outside schools might enter classrooms.