A local milk-truck driver with a childhood grudge and an array of guns Monday took a one-room Amish schoolhouse by storm, sent boys and adults outside, blocked doorways with wood, then opened fire on a dozen girls, killing four in total before committing suicide.

Three girls were pronounced dead soon after the incident. A fourth girl later died at a Pennsylvania hospital, and at least seven victims were still critically wounded, FOXNews confirmed.

It was the nation's third deadly school shooting in less than a week, and it sent shock waves through Lancaster County's bucolic Amish country, a picturesque landscape of horse-drawn buggies, green pastures and neat-as-a-pin farms, where violent crime is virtually nonexistent.

Click here to visit FOXNews.com's Crime Center

Most of the victims had been shot execution-style at point-blank range after being lined up along the chalkboard, their feet bound with wire and plastic ties, authorities said. Two young students were killed, along with a female teacher's aide who was slightly older than the students, state police Commissioner Jeffrey B. Miller said.

"This is a horrendous, horrific incident for the Amish community. They're solid citizens in the community. They're good people. They don't deserve ... no one deserves this," State Police Commissioner Jeffrey B. Miller said.

The gunman, Charles Carl Roberts IV, a 32-year-old truck driver from the nearby town of Bart, was bent on killing young girls as a way of "acting out in revenge for something that happened 20 years ago" when he was a boy, Miller said.

Miller refused to say what that long-ago hurt was. "We're still looking into the motive. I'm not prepared to speak on that yet, there are some things we have to continue to investigate there," he said, possibly including personal trauma Roberts' had experienced in his own family more recently.

Miller also couldn't say why females were chosen, but said that a great deal of planning had gone into Roberts' actions. "It's clear that he did a great deal of planning, just from the list of materials; it appears he intended to be prepared for a lengthy siege and intended to harm these kids and to harm himself," he said.

Roberts had backed up his pickup truck to the entrance of the school, which Miller said struck the teacher as odd, and upon entering the schoolhouse according to Miller Roberts brought with him a 9mm semi automatic pistol he purchased in 2004, a 12 gauge shotgun, a bucket, a change of clothes and a bag with 600 rounds of ammunition, gunpowder, a stun gun, two knives, various tools, wire and rolls of clear tape.

While Miller said he could not speak to Roberts' mental condition by law, he said that from the evidence so far he could glean that even though Roberts put great care into his plan, that the decision to do this was made in the past few days, and may or may not be related to "pressure that he exhibited to co-workers."

Roberts had no criminal history or outstanding warrants, was not Amish and appeared to have nothing against the Amish community, Miller said. Instead, Miller surmised Roberts had apparently picked the school because it was close by, there were girls there, and it had little or no security.

Roberts dropped his children off at school in the morning, Miller said, and his wife left around 8:45am. Roberts then left several rambling notes to his wife and three children at his house that morning that Miller said were "along the lines of suicide notes," and that expressed great anger.

Marie Roberts arrived home after her husband had left and found the notes, and she tried to contact him to no avail, Miller added.

From the suicide notes and telephone calls, it was clear Roberts was "angry at life, he was angry at God," Miller said. And it was clear from interviews with his co-workers at the dairy that his mood had darkened in recent days and he had stopped chatting and joking around with fellow employees and customers, the officer said.

The gunman also called his wife during the siege by cell phone to tell her he was getting even for some long-ago offense, according to Miller.

Miller said that Roberts had been scheduled to take a random drug test on Monday for his job, but that it was not clear what role that may have played in the attack or whether Roberts kept that appointment.

Miller said investigators were looking into the possibility the attack may have been related to the death of one of Roberts' own children. According to an obituary, Roberts and his wife, Marie, lost a daughter shortly after she was born in 1997.

Marie Roberts on Monday released a statement describing her husband as "Loving, supportive, thoughtful — all the things you’d always want and more. He was an exceptional father, he took the kids to soccer practice and games, played ball with them in the back yard and took our 7-year-old daughter shopping." She added in the statement, "He never said no when I asked him to change a diaper."

A family spokesman, Dwight LeFever, read the short statement from Roberts' wife that continued, "Our hearts are broken, our lives are shattered, and we grieve for the innocence and lives that were lost today. Above all, please pray for the families who lost children and please pray too for our family and children."

No one answered the door at Roberts' small, one-story home on Monday afternoon. Children's toys were strewn on the porch and in the yard.

As rescue workers and investigators tromped over the surrounding farmland, looking for evidence around this tiny village about 55 miles west of Philadelphia, dozens of people in traditional plain Amish clothing watched — the men in light-colored shirts, dark pants and broad-brimmed straw farmer's hats, the women in bonnets and long dark dresses.

Reporters were kept away from the school after the shooting, and the Amish were reluctant to speak with the media, as is their custom.

The victims were members of the Old Order Amish. Lancaster County is home to some 20,000 Old Order Amish, who eschew automobiles, electricity, computers, fancy clothes and most other modern conveniences, live among their own people, and typically speak a German dialect known as Pennsylvania Dutch.

Bob Allen, a clerk at a bookstore in the Amish country tourist town of Intercourse, said residents see the area as being safe and the Amish as peaceful people. "It just goes to show there's no safe place. There's really no such thing," he said.

The shooting took place at the one-room West Nickel Mines Amish School, a neat white building set amid green fields, with a square white horse fence around the schoolyard. The school had about 27 students, ages 6 to 13.

According to investigators Roberts walked into the school around 10 a.m. He released about 15 boys, a pregnant woman and three women with babies, Miller said.

He barricaded the doors with two-by-fours and two-by-sixes nailed into place, piled-up desks and flexible plastic ties; made the remaining girls line up along a blackboard; and tied their feet together with wire ties and plastic ties, Miller said.

The teacher and another adult at the school fled to a farmhouse nearby, and someone there called 911 at 10:36am to report a gunman holding students hostage.

Police arrived to the scene at 10:45am, and set up a perimeter while trying to speak to Roberts via loudspeaker. Roberts apparently called his wife around 11 a.m., saying "the police are here" and referring to revenge for an old grudge and that he wouldn't be coming home, Miller said. Their conversation was limited and Miller said Marie Roberts did not hear anything in the background.

Roberts seemed calm if a bit subdued, Miller said, and not agitated. He said Roberts went in alone, planned to go in alone, and it seemed he had no intention of coming out alive, "whether by his own hand or in a police suicide scenario."

Moments later, Roberts called 911 himself and told a dispatcher he would open fire on the children if police didn't back away from the building. Miller said negotiators obtained his cellphone number from that call and scrambled to get ahold of him, but within seconds, troopers heard gunfire. They smashed the windows to get inside, and found his body and those of the children.

Miller said he had no immediate evidence that the victims were sexually assaulted or that there was any attempt at a sexual assault.

Killed were two students, and a female teacher's aide who was 15 or 16 years old, authorities said. Another young student died later Monday night. Police did not reveal the identities or exact ages of the victims, saying Monday evening that 2 had yet to be identified.

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

Miller, though, said he believed the Pennsylvania attack was not a copycat crime: "I really believe this was about this individual and what was going on inside his head."

On Friday, a school principal was shot to death in Cazenovia, Wis. A 15-year-old student, described as upset over a reprimand, was charged with murder.

The Pennsylvania attack was the deadliest school shooting since a teenager went on a rampage last year on an Indian reservation in Red Lake, Minn., killing 10 people in all, including five students, a teacher, a security guard and himself.

Nationwide, the 1999 Columbine High School massacre in Littleton, Colo., remains the deadliest school shooting, with 15 dead, including the two teenage gunmen.

In Pennsylvania's insular Amish country, the outer world has intruded on occasion. In 1999, two Amish men were sent to jail for buying cocaine from a motorcycle gang and selling it to young people in their community.

There were four murders in Lancaster County in 2005, including the killings of a non-Amish couple were shot to death in their Lititz home in November by their daughter's 18-year-old boyfriend.

Kenneth Trump, president of the National School Safety and Security Services consulting firm in Cleveland, said the Colorado and Pennsylvania crimes underscore the lesson that no school is automatically safe from an attack.

"These incidents can happen to a one-classroom schoolhouse to a large urban school," he said. "The only thing that scares me more than an armed intruder in a school is school and safety officials who believe it can't happen here."

Click here to visit FOXNews.com's Crime Center

FOXNews.com's Megan Manni and The Associated Press contributed to this report.