Why would someone do that? That is one of the first questions that many people ask after a mass shooting.
Adam Lanza, 20, killed 20 children and 6 adults at Sandy Hook Elementary in Newton, Conn. on Friday. His mother was a teacher at the school and was among those killed.
"It's very hard to know anything specific because we don't have facts," said Dr. John Oldham of Baylor College of Medicine.
Psychology experts said it's hard to know what Lanza's state of mind was before the shootings.
"We can say that this clearly is an act of desperate rage," Oldham said. "The perpetrator could kill children in addition to the adults and not hesitate to do that."
While the motive for the shooting is not clear, Oldham said he would not be surprised in Lanza had a history of violence or psychotic behavior.
"For someone to carry out an attack like this, clearly they have been suffering and dealing with some kinds of problems," Oldham said.