A girl of 14 shot and wounded a female classmate before apparently taking her own life in an attack on a Texas high school.

The 'straight-A student' opened fire on another girl at Alpine High in an attack that sent hundreds of pupils fleeing for safety.

She was later found lying inside the building alongside her semi-automatic gun and a large supply of ammunition, sheriff Ronnie Dodson told NBC KWES news channel.

"We're still in shock," the sheriff told a news conference.

The female victim, who was reportedly shot in the "lower extremities", is being treated in hospital for non-life-threatening injuries.

A homeland security officer was also injured after he was accidentally shot in the leg by "friendly fire" from a US marshall as they responded to the incident.

Mr Dodson said that the teenage attacker had moved to the small town of Alpine only six months before the shooting.

Her grandparents described her as a diligent student and said there were no signs that she was troubled, he added.

The incident sent all three schools in the district into lockdown.

Police also said they were investigating bomb threats sent to the town's hospital and the nearby Sul Ross State University shortly after the shooting, but both threats were thought to be hoaxes.

Some of the high school's students were seen comforting each other after they were evacuated to a nearby church.

Some also posted messages on Twitter thanking police and expressing shock at the violent attack.