MEXICO CITY (REUTERS) - A teenage student suffering from depression opened fire at a private American school in northern Mexico on Wednesday (Jan 18), injuring three students and a teacher then shooting himself in what state officials called an unprecedented attack that was caught on security video.

Aldo Fasci, security spokesman for Nuevo Leon state, said the 15-year-old student pulled out a handgun inside a classroom at the bilingual Colegio Americano del Noreste and began shooting, leaving three of the victims and himself gravely injured.

Security camera footage showed the teenager quickly and calmly firing what appeared to be seven shots at seated students and a female teacher, some at point blank range.

At least two victims immediately slumped over after being hit.

The shooter, looking dazed, aimed at his own temple and pulled the trigger twice, but he had apparently ran out of bullets. He walked back to where he had been sitting, reloaded, and shot himself in the chin. He keeled over.

Students who had been cowering beneath desks and chairs then fled the classroom, stepping over the shooter to reach the door.

"He had depression and was being treated," Fasci told local television. "We have no motive yet."

Ambulances and police raced to the school, which offers pre-school, elementary and high school classes on the southern outskirts of the industrial city of Monterrey.

Fasci said one 15-year-old student was shot in the arm, and two 14-year-olds were shot and injured. There were no details on the nationalities of the victims. He said the shooter had brought a .22 caliber firearm from home.

Oscar Aboytes, spokesman for emergency services in Nuevo Leon, said all of the wounded were rushed to hospital.

Photos widely shared on social media showed three people lying on a classroom floor with pools of blood around their heads. Reuters was not able to confirm the authenticity of those images.