Story Highlights

• NEW: Fifth girl dies in hospital; six others in critical condition
• Truck driver barricaded self in Amish school before shootings, suicide
• Notes left to family indicate motive was two-decade-old grudge
• Assortment of weapons and tools suggest preparation for long siege

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PARADISE, Pennsylvania (CNN) -- A heavily armed truck driver who was prepared for a long standoff barricaded himself in a one-room Amish schoolhouse Monday, killing five girls execution-style before killing himself, police said.

Six other girls were bound and critically wounded in the attack, which police said appeared to be a revenge killing for an unspecified incident that occurred when the gunman was a boy.

The attack was the nation's third deadly school shooting in a week. (Watch what happened in schoolhouse, community's reaction -- 2:21 )

The gunman, 32-year-old Charles Carl Roberts IV, carried three firearms -- a shotgun, a semiautomatic pistol and a rifle. State Police Commissioner Jeffrey Miller said Roberts also had a stun gun, two knives and a bag holding 600 rounds of ammunition.

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 siege.

"It seems as though he wanted to attack young female victims, and this is close to his residence. That's the only reason we can figure that he went to the school," said Miller. (Watch commissioner's dramatic account of shootings -- 7:44 )

"It's a horrendous crime scene," he said.

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

The school teaches students ranging from 6 to 13, with some older students working as teachers' aides. Two students and an aide were killed, one of them dying in a trooper's arms, Miller said. Two other girls died while hospitalized hours after the shooting.

The wounded girls were taken to several hospitalsin the region, some of them with gunshot wounds to the head.

Roberts drove a commercial milk truck, said Miller. He was not Amish, was not previously wanted by authorities and had no known criminal history.

The Bart, Pennsylvania, resident had three children, and left several notes for his family "along the lines of suicide notes," Miller said.

The notes were "rambling" and "didn't make much sense," Miller said. (Read Miller's remarks)

Roberts did not respond when police at the scene attempted to communicate with him, and troopers were preparing to storm the schoolhouse before the shooting began, Miller said.

Roberts' wife tried to call him when she found the notes, Miller said. He returned the call on his cell phone, told her he wouldn't be coming home and "that he was acting out to achieve revenge for something that happened 20 years ago."

The suspect called 911 and "made a declaration that if the state police didn't leave the property he would start shooting people within 10 seconds."

"Within seconds, he just started firing in rapid succession."

Police then stormed the schoolhouse through the windows and found Roberts dead, Miller said. He had fired at least one shotgun blast at police.

Gunman let boys go

The commissioner said Roberts had worked a shift that ended about 3 a.m. Monday and taken his children to their bus stop before beginning his rampage.

Police said he had barricaded himself in the schoolhouse with two-by-four and two-by-six pieces of lumber, which he brought to the scene in a borrowed pickup truck.

"He planned this out meticulously," the commissioner said.

Miller said that during the attack, Roberts apparently told students to line up in front of the blackboard. He began to tie his female captives' feet together. He let the 15 boys in the classroom leave.

Miller said Roberts also allowed an adult female who was pregnant to leave with three other adult females who had infant children with them.

The teacher who was released was able to alert authorities, Miller said.

"Apparently, there was some sort of an issue in his past that for some reason, he wanted to exact revenge against female victims. That's all I can tell you right now," Miller said.

Miller said Roberts' grudge did not appear to involve the Amish community and that he may have chosen his target out of convenience, perhaps thinking "getting into a school like this was maybe just a little bit easier."

A man who said he was a friend of the Roberts family, Dwight Lefever, read a statement attributed to the gunman's wife, Marie Roberts, according to CNN affiliate WGAL.

"The man that did this today was not the Charlie I've been married to for almost 10 years. My husband was loving, supportive, thoughtful. All the things you'd always want and more. He was an exceptional father," the statement said.

"Our hearts are broken, our lives are shattered, and we grieve for the innocents and lives that were lost today. Above all, please pray. Pray for the families who lost children today, and please pray, too, for our family and children."

Lancaster County's 911 Web site reported dozens of emergency vehicles -- including at least nine ambulances -- were dispatched to Bart Township shortly before 11 a.m. for a "medical emergency." The Lancaster County emergency communications Web site showed 20 incident calls listed to the normally quiet Bart Township at 10:48 a.m.

Shortly after the attack, men, women and girls in traditional Amish clothing gathered in small groups outside the country schoolhouse along a dirt road. Horse-drawn carriages could be seen in a nearby parking lot.

The school is one of about 150 that serve the Amish population of about 200,000, said Donald Kraybill, a sociology professor at nearby Elizabethtown College.

"They typically are one-room facilities operated by Amish parents," Kraybill said. "There usually are not any security concerns or security risks in the schools, so this is quite a shock."

Aaron Meyer, who owns a buggy company in the nearby village of Bird in Hand, said the largely rural Amish country had experienced nothing like the crime in the 25 years he has lived in the area.

"Many of these townships there have no police at all, because there's no crime," Meyer told CNN.

A White House spokesperson said the president was "deeply saddened and troubled by the recent school violence" and that the administration would convene a conference on the subject next week. (Full story)

On 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 SWAT team stormed the classroom. (Full story)

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