Carrying a change of clothes and toilet paper, Roberts planned for a long stay inside the one-room schoolhouse, but it ended quickly when police showed up. Roberts opened fire on 10 tied-up little girls, killing five of them, and then killed himself.
Authorities on Tuesday laid out the details of a disturbing plot by Roberts — a man who said he was tormented about molesting two relatives 20 years ago and by dreams of doing it again. Police also raised the possibility that Roberts, who brought lubricating jelly with him, may have been planning to sexually assault the Amish girls.
"It's very possible that he intended to victimize these children in many ways prior to executing them and killing himself," State Police Commissioner Jeffrey B. Miller said. But Roberts became disorganized when police arrived, and shot himself in the head, Miller said.
"I don't think you can make sense of it. My younger child last night had problems with this," Miller told CBS News national correspondent Byron Pitts. "You don't want children to be scared to go to school. You think in America if there's anywhere in the world you'll be safe it would be in a one-room Amish schoolhouse."
Roberts left separate notes for his wife and each of his three children, who are all 6 years or younger, at their home in Bart, Miller said.
Roberts also said he was haunted by the death of his prematurely born daughter in 1997. The baby, Elise, died 20 minutes after being delivered, Miller said.
Elise's death "changed my life forever," the milk truck driver wrote to his wife. "I haven't been the same since it affected me in a way I never felt possible. I am filled with so much hate, hate toward myself hate towards God and unimaginable emptyness it seems like everytime we do something fun I think about how Elise wasn't here to share it with us and I go right back to anger."
The state police commissioner laid out in chilling detail the steps Roberts took in the days and hours leading up to his attack Monday morning on the West Nickel Mines Amish School in Lancaster County, where the Amish live a peaceful existence in an 18th-century world with no automobiles and electricity.
"He certainly was very troubled psychologically deep down and was dealing with things that nobody else knew he was dealing with," Miller said.
During the standoff, Roberts told his wife in a cell phone call from the one-room schoolhouse that he molested two female relatives when they were 3 to 5 years old, Miller said. Also, in the note to Marie Roberts, he said he "had dreams about doing what he did 20 years ago again," Miller said.
Police could not immediately confirm Roberts' claim that he molested two relatives. Family members knew nothing of molestation in his past, Miller said. Police located the two relatives and were hoping to interview them.
At the time Roberts' wife received the phone call, she was attending a meeting of a prayer group she led that prayed for the community's schoolchildren.
Roberts, who was not Amish and did not appear to have anything against the Amish, had planned the attack for nearly a week, buying plastic ties from a hardware store on Sept. 26 and several other items less than an hour before entering the school, Miller said.
Using a checklist that was later found in his pickup truck, Roberts brought to the school three guns, a stun gun, two knives, a pile of wood for barricading the doors, and a bag with 600 rounds of ammunition, police said.