NEWTOWN, Conn. — The massacre of 26 children and adults at a Connecticut elementary school elicited horror and soul-searching around the world even as it raised more basic questions about why the gunman, a 20-year-old described as brilliant but remote, was driven to such a crime and how he chose his victims.

Investigators were trying to learn more about Adam Lanza and questioned his older brother, who was not believed to have been involved in the rampage at Sandy Hook Elementary. Police shed no light on the motive for the second-deadliest school shooting in U.S. history.

Dr. Michael Teague, a forensic psychologist, has worked with the FBI and SBI to figure out why criminals commit such unimaginable acts.

"What we are doing is psychological autopsy," Teague said Saturday morning.

While he is not working on the Newtown case, Teague sees it as a probable personality disorder. The shooter killed young children because his own life was so miserable.

"I think he was thinking, 'These poor kids, they are getting ready to come up and get older and be miserable and so I've got to go down there and take them out of their misery,'" Teague said.

"In his mind, he was not killing them. It was mercy killing in his mind," Teague said.

Law enforcement officials in Connecticut have said Lanza is believed to have suffered from a personality disorder and lived with his mother. Authorities said he had no criminal history.

Asked at a news conference whether Lanza had left any emails or other writings that might explain the rampage, state police Lt. Paul Vance said investigators had found "very good evidence" and hoped it would answer questions about the gunman's motives. Vance would not elaborate.

However, another law enforcement official, speaking on condition of anonymity, said that investigators have found no note or manifesto of the sort they have come to expect after murderous rampages.

Key facts:

 THE VICTIMS: The chief medical examiner said all the victims at the school were shot with a rifle, at least some of them up close, and all of them apparently were shot more than once. More Info     Victims of the Newtown, Conn., shooting

All six adults killed at the school were women. Of the 20 children, eight were boys and 12 were girls. All the children were 6 or 7 years old.

Among the dead: popular Principal Dawn Hochsprung, who town officials say tried to stop the rampage and paid with her life; school psychologist Mary Sherlach, who probably would have helped survivors grapple with the tragedy; a teacher thrilled to have been hired this year; and a 6-year-old girl who had just moved to Newtown from Canada.

THE GUNMAN: Adam Lanza, 20, was described as a bright but painfully awkward student who seemed to have no close friends. In high school, he was active in the technology club. The club adviser remembered that he had "some disabilities" and seemed not to feel pain like the other students. That meant Lanza required special supervision when using soldering tools, for instance. He also had an occasional "episode" in which he seemed to withdraw completely from his surroundings, the adviser said.

Authorities said Lanza had no criminal history, and it was unclear whether he had a job.

Lanza committed suicide after his killing spree.

THE PARENTS: One of the parents who lost a child in the attack spoke publicly about his loss.

Robbie Parker fought back tears and struggled to catch his breath as he described his 6-year-old daughter, Emilie, as a little girl who loved to draw. He also reserved surprisingly kind words for the gunman, saying he was not mad and offering sympathy for the gunman's family.

To the man's family, he said, "I can't imagine how hard this experience must be for you."

In a statement released late Saturday, the father of the gunman said his family is struggling to make sense of what happened.

Peter Lanza said the family "is grieving along with all those who have been affected by this enormous tragedy."

THE INVESTIGATION: Police shed no new light on what triggered the shooting, but state police Lt. Paul Vance said investigators had found "very good evidence" that they expected to use "in painting the complete picture." He would not elaborate.

Newtown education officials said they had found no link between Adam Lanza's mother, who was killed before the school massacre, and the school, contrary to news reports that said she was a teacher there. Investigators believe Adam Lanza attended Sandy Hook Elementary many years ago, but they had no explanation for why he went there Friday.

THE SCENE: Some residents of Newtown, a picturesque New England community, began taking down Christmas decorations in the wake of the slayings. Signs around town read, "Hug a teacher today," ''Please pray for Newtown" and "Love will get us through."

When the list of the victims was released, nearly everyone already seemed to know someone who had died.

THE GUNS: Federal authorities visited local gun ranges but found no evidence that the gunman trained for the attack or was an active member of the recreational gun community.

Investigators also have interviewed gun dealers trying to determine whether there was any training or other behavior that precipitated the attack.

The HISTORY: The Newtown massacre is the second-deadliest school shooting in U.S. history and one of the deadliest mass shootings around the world. A gunman at Virginia Tech University killed 33, including himself, in 2007. Only Virginia Tech and the mass killings of 77 in Norway last year had greater death tolls across the world over the past 20 years.