The truck driver who opened fire inside a tiny Amish schoolhouse in Pennsylvania Monday, killing four students, sent the boys outside before tying up the girls and shooting them at close range, police say.

The gunman, who shot himself to death, had three guns, a stun gun, two knives and a bag with about 600 rounds of ammunition, police said.

An Amish buggy passes law enforcement personnel in Nickel Mines, Pa., on Monday. An armed man walked into an Amish school, sent the boys outside and tied up and shot several of the girls, authorities said.
(USA TODAY, Eileen Blas/ AP)

Seven other girls, most badly wounded, were transported to hospitals in the region. They had been shot execution-style at point-blank range after being lined up along a chalkboard, their feet bound with wire and plastic ties, authorities said.

Police identified the shooter as Charles Carl Roberts, 32, a married father of three who drove a milk tanker truck in the southeastern Pennsylvania community where the shooting took place.

He was not Amish, police say, but left notes indicating he was "acting in a way to achieve revenge for something that happened 20 years ago."

(CBC)

The shooter's wife said she last saw her husband about 8:45 a.m. local time, when he walked his children to a school bus stop. Roberts worked a night shift delivering milk from farms to a processing plant.

His wife briefly left their home and when she returned, she found suicide-style notes to her and the three children, police said.

At about 11 a.m., he called his wife from inside the school, located about 10 kilometres from their house, and told her he wouldn't be coming home.

Girls targeted in attack

Roberts appeared to want to attack young female victims, state police Commissioner Jeffrey Miller said at a Monday afternoon news conference.

Roberts ordered the boys out and bound the girls by the blackboard using plastic cuffs and wire, police said.

He then used two-by-fours to barricade the doors of the schoolhouse.

A police negotiator wasn't able to make contact with the shooter, who fired out the windows of the schoolhouse.

"Obviously these schools are not like most public schools that would have some type of security," Jack Lewis of the Pennsylvania State Police told CBC News in an interview shortly after the shooting.

The school is in the community of Nickel Mines, about 100 kilometres west of Philadelphia, in the heart of what is known as Pennsylvania Dutch Country.

It is heavily populated by people observing the Amish faith, a type of conservative Christianity that emphasizes plain living and forbids acts of violence.

Third American school shooting in past week

It was the third small community in the United States to be shocked by a deadly school shooting in less than a week.

On Friday, a school principal was gunned down in Cazenovia, Wis. A 15-year-old student, who has been described as upset over a reprimand, has been charged with murder in the killing.

Just two days earlier, an adult gunman held six girls hostage in a school at Bailey, Colo., before killing a 16-year-old girl and then himself.

Canada was also shocked by a school shooting last month.

On Sept. 13, an 18-year-old student was killed when an armed man entered Dawson College in Montreal's downtown and opened fire, killing her, wounding 19 others and then turning the gun on himself.

The Pennsylvania tragedy also raises memories of the December 1989 shooting at L'École Polytechnique in Montreal. The shooter, Marc Lépine separated the men from the women, telling the men to leave. He killed 13 women students and a college employee before killing himself.

With files from the Associated Press