Early Tuesday, authorities confirmed reports that a fourth girl has died.
The 7-year-old girl died about 4:30 a.m. at Penn State Children's Hospital in Hershey, hospital spokeswoman Amy Buehler Stranges said.
"Her parents were with her," Buehler Stranges said. "She was taken off life support and she passed away shortly after."
Six other girls are in the hospital.
At Penn State Children's Hospital, a 6-year-old girl is in critical condition and a 13-year-old girl is in serious condition. Another victim is at Christiana Hospital in Delaware, where officials are not releasing any information. And three girls, ages 8, 10 and 12, are at Children's Hospital of Philadelphia, where a spokeswoman says they are out of surgery but are in critical condition.
Police say notes and phone calls show the gunman, Charles Carl Roberts IV, 32, was "angry at life, he was angry at God." While some who knew him saw no signs of trouble, others say his mood had darkened and he'd stopped chatting and joking with co-workers and customers.
This is America's third deadly school shooting in less than a week.
CBS News correspondent Mark Knoller reports President Bush, reacting to the shootings, has ordered the U.S. Attorney General and the Secretary of Education to study ways the government can help prevent school violence.
A conference on that subject is planned for next week with the participation of law enforcement, educators and parent groups.
Monday's rampage shattered the typical quiet of Pennsylvania's Lancaster County, home to the Amish, horse-drawn buggies, green pastures and neat-as-a-pin farms, where violent crime is rare.
A teacher called police around 10:30 a.m. and reported that a gunman was holding students hostage.
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.
Click here to read about the U.S. Department of Education's school safety initiatives. Keep Schools Safe provides additional resources for parents on school safety issues.
"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.
Miller says Roberts, who lived in the nearby town of Bart, Pa., 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 refused to say what that long-ago hurt was.
A family spokesman, Dwight LeFever, read a short statement from Roberts' wife.
"The man who did this today was not the Charlie I have been married to for almost ten years. My husband was loving, supportive, thoughtful... all the things you would always want and more," she said. "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."
Neighbors who knew the Roberts' family said they saw no indications of trouble brewing.
"They're a fine Christian family. It's ironic and it's heartbreaking," said Lois Fiester, a relative, as she stood outside the family's modest tan ranch house.
Police say Roberts was not Amish, appeared to have nothing against the Amish, and apparently chose the school because it was close by, there were girls there, and it had little or no security.
The attack bore similarities to a deadly school shooting last week in Bailey, Colorado, and authorities there raised the possibility that the Pennsylvania attack was a copycat crime.