At least two female students and one slightly older female — who may have been a teacher's aide — were shot and killed execution style in what police say may be a revenge killing for something that happened 20 years ago.

State Police Commissioner Jeffrey Miller said Monday that truck driver Charles Carl Roberts IV entered the one-room Amish schoolhouse in Lancaster County earlier in the day. The gunman brought in pieces of wood to board up the doors to the building when he entered the school, and also stacked desks against the building's doors.

"It appears that the suspect entered the school with the intention of taking hostages," Miller said.

Roberts had wire ties and plastic flex cuffs, which he used to begin tying the hostages feet together after they were lined up along a blackboard. He let go 15 male students, as well as one adult female who was pregnant. Three other non-student females with infants were also allowed to leave. One shotgun and one handgun was found next to the suspect when police entered the building.

"It seems as though he wanted to attack young, female victims," Miller said.

After a half hour inside, Roberts began shooting. When the gunfire stopped, three females were found shot dead at the scene, along with the suspect, who shot himself, Miller said. At least seven more victims — some critically wounded — were transported to nearby hospitals, he said. One victim, a young girl, died in the arms of a trooper, Miller said. He could not confirm how many people died en route or at the hospital. The county coroner earlier reported six children killed but then backed off of that statement.

There were also notes left by Roberts, a 32-year-old milk delivery man from nearby Bart, Pa., Miller said. The notes, left for Roberts' wife, indicated that he was seeking revenge for something that happened decades ago. Roberts had three children.

Although Miller said police had not yet thoroughly processed the school where the shootings took place, he described it as a "horrendous crime scene."

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It was the nation's third deadly school shooting in less than a week, and one of the deadliest in U.S. history.

Kim Hatch, spokeswoman for the Lancaster General Hospital, said three victims ages six through 12 arrived there with gunshot wounds — some to the head — and are in critical condition. Two of those victims were sent to Penn State Milton S. Hershey Medical Center, while the third was sent to the Children's Hospital of Philadelphia. One other patient has been sent to Reading Hospital of Pennsylvania.

John Line, also from Lancaster General, told FOX News that their medical center doesn't expect to receive anymore patients. Wolf Rock School, where the shooting took place, is in Paradise Township on Mine Road — an Amish community within the township. There are a total of 27 students in grades 1 through 8 enrolled at the school.

Miller said the first emergency call came in at 10:36 a.m. Monday morning, from a teacher who was freed from the school. The call said a male gunman had entered the school and taken hostages. There had been 15 males ages six through 13 in the room, along with 10 to 12 female students of the same age. A teacher, along with some teacher's aides — who are normally slightly older than the students — were also in the room when Roberts entered.

Miller said state troopers first arrived to the scene and set up a perimeter at 10:45 a.m. The shooter had boarded up all the doors and stacked desks against the building's entrances.

Police could not communicate with the shooter until after a 911 call from a county facility came in, saying the gunman had called and said if police didn't leave the school grounds within 10 seconds, he would begin shooting the students.

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.

Police then had the gunman's cell phone number after he placed that call, and tried to contact him. Within a few seconds though, Miller said, police heard "multiple shots in quick succession." By the time officers broke windows to get in, three girls and the gunman were dead, Miller said.

"It was that quick. We're trying to get the word out to do a coordinated assault and he starts firing — at the same time our negotiator is trying to get him on his cell phone," Miller said.

It appears the gunman had an automatic handgun as well as a shotgun in his possession, Miller added.

Police late Monday morning surrounded the one-room schoolhouse, a tiny building surrounded by a white fence and farm fields in southeastern Lancaster County. The Lancaster County 911 Web site reported that dozens of emergency units were dispatched to a "medical emergency" at 10:45 a.m. Monday.

Hours after the attack, about three dozen people in traditional Amish clothing, broad-brimmed hats and bonnets stood nearby speaking to one another and authorities. Others gathered with a group of children at a nearby farm. Emergency responders were seen searching the school grounds for evidence.

The school is just outside Nickel Mines, a tiny village about 55 miles west of Philadelphia.

Two other school shootings occurred last week. 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. Click here for the latest on the Cazenovia story.

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. Authorities said the man sexually molested the girls. Click here to read the latest on that story.

"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."

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. Last year, a 16-year-old shot seven people to death at a school on Minnesota's Red Lake Indian Reservation, then killed himself.

FOXNews.com's Liza Porteus and The Associated Press contributed to this report.