Charles Carl Roberts IV entered the class and ordered all the boys and some adults to leave. He then lined up the girls in front of the blackboard, tied their feet using wire or plastic cuffs, and shot them in the head.

The gunman was armed with three guns, a stun gun, two knives and a bag holding 600 rounds of ammunition, said Commissioner Jeffrey Miller of the Pennsylvania State Police. He was also carrying an assortment of tools and other items, including toilet paper that led police to believe he was prepared for an extended standoff.

The shooting took place in Lancaster County, about 60 miles west of Philadelphia, at a one-room school run by the Amish community. There were 26 students in the school on Monday, 11 of them were girls.

The police said the attack appeared to be a revenge killing for an incident that occurred two decades ago. It was the third shooting at a US school in the past week.

Last Wednesday, a 53-year-old man entered a high school in Bailey, Colorado, where he held several female students hostage at gunpoint. He shot one girl before killing himself seconds after a Special Weapons and Tactics team stormed the classroom.

A high school student near Madison, Wisconsin, is suspected of fatally gunning down his principal last Friday, after he complained about being bullied and was disciplined for carrying tobacco.