NICKEL MINES, Pa. – A 32-year-old milk truck driver took about a dozen girls hostage in a one-room Amish schoolhouse Monday, barricaded the doors with boards and killed at least three girls and apparently himself, authorities said.

It was the nation’s third deadly school shooting in less than a week, and similar to an attack just days earlier at a school in Colorado.

The gunman, identified as Charles Carl Roberts IV, was inside for over half an hour and had barred the doors with 2x4s with the girls inside, State Police Commissioner Jeffrey B. Miller said. By the time officers broke windows to get in, three girls and the gunman were dead, Miller said. Seven others were taken to hospitals, three in critical condition.

“It appears that when he began shooting these victims, the victims were shot execution style in the head,” Miller said.

Roberts had walked into the one-room West Nickel Mines Amish School with a shotgun and handgun, then released about 15 boys, a pregnant woman and three other women with infants before barring the doors with the girls inside, Miller said.

The girls were lined up along a blackboard, Miller said. “He had wire ties with him and flex ties, and he began to tie the girls’ feet together,” Miller said.

A teacher was able to call police around 10:30 a.m. and reported that a gunman was holding students hostage.

About 11 a.m., Roberts apparently called his wife from a cell phone, saying he was “acting out in revenge for something that happened 20 years ago,” Miller said. “It seems as though he wanted to attack young, female victims.”

Moments later, Roberts told a dispatcher he would open fire on the children if police didn’t back away from the building. Troopers heard gunfire in the building seconds later.

Miller released no further details about that what the grudge Roberts mentioned could have involved.

The school has about 25 to 30 students in all, ages 6 to 13.

Lancaster County Coroner G. Gary Kirchner initially said six people were killed, but later said he wasn’t certain about that number.

At least seven people were taken to hospitals, including at least three girls, ages 6-12, who were admitted to Lancaster General Hospital in critical condition with gunshot wounds, spokesman John Lines said.

The small school, surrounded by a white board fence, sits among farmlands just outside Nickel Mines, a tiny village about 55 miles west of Philadelphia.

Hours after the attack, about three dozen people in traditional Amish clothing, broad-brimmed hats and bonnets stood near the small schoolhouse as investigators walked in a line through fields searching for evidence.

The shootings were disturbingly similar to an attack last week at Platte Canyon High School in Bailey, Colo., where a man took several girls hostage in a school classroom and then killed one of them and himself. Authorities said the man sexually molested the girls.

“If this is some kind of a copycat, it’s horrible and of concern to everybody, all law enforcement,” said Monte Gore, undersheriff of Park County, Colo.

“On behalf of Park County and our citizens and our sheriff’s office, our hearts go out to that school and the community,” he said.

Nationwide, the 1999 Columbine High School massacre in Littleton, Colo., remains the deadliest school shooting, claiming the lives of 15 people, including the two teenage gunmen. On Friday, a school principal was gunned down in Cazenovia, Wis. A 15-year-old student, described as upset over a reprimand, was charged with murder in the killing.