Pennsylvania police said the man lost a child of his own three years ago, and they were investigating suicide notes that he left as well as statements he made to his wife that he was seeking revenge for something that occurred 20 years ago.

Two girls died overnight, bringing to five the number of students killed, police said on Tuesday.

The two girls who died overnight were aged 7 and 8, said Commissioner Jeffrey Miller of the Pennsylvania state police. One other girl remained in critical condition and another four were described as stable, state police said.

Commenting on Roberts' motive, Miller said the death of a child three years ago may have affected his state of mind.

"He may have been angry with God for having lost a child," Miller said at a news briefing.

The third deadly U.S. school shooting in a week shattered the calm of an Amish farm community where there is little crime and where the sight of horse-drawn buggies, men in simple black coats and hats, and women in bonnets conjures up a bygone age.

"We don't believe that he had ... some animus toward the Amish community. We believe that this was a target of opportunity," Miller said. "He believed he could get in there pretty easily and secure it from a defensive posture."

Several members of the Amish community interviewed by Reuters said they were sad and disappointed but not angry and emphasized the need for forgiveness.

"It's just not the way we think. There is no sense in getting angry," said Henry Fisher, 62, a retired farmer with five grown children and 33 grandchildren who has lived all his life in the town some 60 miles west of Philadelphia.

He said the Amish lifestyle with no cars, television or credit cards, was "a more peaceful life ... to keep the next generation living a more humble life."

He also said he did not expect additional security such as locks on schools because this was a "freak accident."

Descendants of Swiss-German settlers, the Amish live in communities familiar to many from the 1985 movie "Witness" starring Harrison Ford as a detective trying to protect an Amish boy threatened by mobsters after he witnesses a crime.

A 25-year-old Amish man who declined to give his name said he lost his 13-year-old niece in the shooting and another niece aged 11 was in stable condition in a Philadelphia hospital.

He expressed resignation rather than anger. "I think it was going to happen. God has his hand in it," he said.

The gunman, identified as Charles Carl Roberts, 32, was not Amish and had no prior criminal record.

According to the Lancaster Era newspaper, Roberts and his wife, Marie Roberts, had a daughter who died as an infant.

They also have two sons and a daughter, ranging in age from 1-1/2 to 7, the paper said. Roberts walked his older children to a school bus stop in the morning, waited until his wife left the house, left suicide notes for his family and then drove a borrowed pickup truck to the Amish school.

He dismissed the boys in the school, as well as the teacher and some other adults. Surrounded by police around 45 minutes after he entered the school, he made brief calls by cellphone to his wife and to police and then opened fire on his victims.

Roberts fired three rounds from a shotgun and 13 from a 9mm semiautomatic pistol, police said, shooting the children "execution style" in the back of the head.

"Death of Innocents" was the headline in the Intelligencer Journal of Lancaster, Pennsylvania, which quoted Marie Roberts as saying her husband was "loving, supportive, thoughtful, all the things you'd always want and more."

She was also quoted as saying he was "an exceptional father" who took his children to soccer practices, played ball in the backyard and took their 7-year-old daughter shopping. Reuters