State Police Commissioner Jeffrey Miller said 32 year-old Charles Roberts was also "angry with God," hated himself over the death of his premature infant daughter nine years ago and dreamed of molesting again.
The sordid revelations came hours after two Amish girls aged seven and eight died of their wounds after they were shot "execution-style" in the head in the rampage which shattered the idyllic rural village.
Among the latest details, police revealed a nine-year-old managed to escape before Roberts started shooting.
It was the third fatal shooting in a US school in a week and five girls from the peace-loving Amish community were still battling for life after the attack.
Meticulous planning
Police said Roberts took wood, latches and a lubricant to the one-room Pennsylvania schoolhouse, indicating intent to restrain and molest the schoolgirls before killing them.
"With the kind of wood and I-bolts and flex cuffs and KY jelly and other things ... it's very possible that he intended to victimize these children in many ways prior to executing them and killing himself," said the police chief.
Roberts confessed to the past sexual assaults in a mobile phone call to his wife minutes before the slaughter, said Commissioner Miller.
"He said 'I am not coming home,' and he states 'I molested some minor family members that were three or four years old, 20 years ago'".
"In his suicide note, he mentioned he was having dreams of molesting again," Miller said.
There was no evidence schoolhouse victims were molested before they were cut down in a hail of bullets which ended when Roberts turned the gun on himself.
The killer's wife said he was also traumatized by the loss of an infant daughter who lived for just 20 minutes after birth nine years ago.
"Roberts was angry with God for taking (her) as outlined in the suicide note," Miller said.
Death toll rises
The latest victim, a seven-year-old, died after being removed from life support at a Pennsylvania hospital. Police said the other girl died in a hospital in Delaware state.
Three girls, aged eight, 10 and 12, underwent surgery for gunshot wounds at Children's Hospital in Philadelphia, where they remain in critical condition.
Two other survivors remain at the Hershey Medical Centre -- a six-year-old in critical condition and a 13-year-old listed in serious condition.