NEW HAVEN, Conn. (AP) — The man who killed 26 people inside a Connecticut elementary school last year showed interest in other mass killings, people close to the investigation told The Associated Press.

The 20-year-old gunman, Adam Lanza, killed his mother at their home before killing 20 first-graders and six staff members at Sandy Hook Elementary School in Newtown on Dec. 14. He killed himself as police arrived.

Authorities found literature on other mass shootings at Lanza's house, according to a law enforcement official briefed on the investigation. Another person familiar with the investigation says Lanza demonstrated an interest in other mass murders. They spoke on condition of anonymity because the investigation is continuing.

The discovery suggests Lanza didn't act on impulse and might have used past mass killings as a guide.

"It certainly lends some evidence of prior planning and at least a fascination with these kind of incidents, if not using it as a way to sort of develop a plan," said Jack McDevitt, associate dean in the School of Criminology and Criminal Justice at Northeastern University.

Other killers have been found with materials from earlier mass shootings or cited the crimes, said Jack Levin, a professor at Northeastern University who has written a number of books on mass murderers.

"What it indicates to me is that he had mass murder in his mind and he was looking for some role models and he quite easily found them in the publicity that had been given to other cases," Levin said.

The massacre at Columbine High School in 1999 has been cited by later killers in the United States and other countries, he said.

"The copycat phenomenon thrives on excessive publicity and we have contributed a great deal by displaying excessively these horrific crimes in our popular culture," Levin said. "The copycat phenomenon doesn't cause the event to happen. It determines the timing and the method."

State police have declined to comment and authorities have not provided a possible motive for the Newtown shooting. A police report is expected around June.

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The Hartford Courant and the Hearst Connecticut Media Group first reported Wednesday that Lanza had done research on other mass killings.

The Courant previously reported that investigators found news articles about Norwegian mass murderer Anders Behring Breivik at Lanza's Newtown house. Breivik killed 77 people in twin attacks in 2011 in Norway's worst peacetime massacre. Several media outlets, including CBS, reported Lanza may have been trying to top the Norway shooter.

Lanza, whose mother used to take him to shooting ranges, killed all of his victims at the schoolhouse with a Bushmaster semi-automatic rifle that he took from the house where he lived with Nancy Lanza. He used a handgun to kill himself.

Jeffrey Fagan, a Columbia law professor, said he doubted the materials planted the idea in Lanza's head but could have given him guidance on tactics. He said it's extremely difficult if not impossible to predict the effect of the materials.

"A would-be shooter could be just as likely to be turned off by exposure to the life histories or tactical details of other mass shootings, or those same details could push an unstable person from passive reading to active planning and ultimately action," Fagan wrote in an e-mail. "This is beyond finding a needle in a haystack, it's more like finding a speck of dust."

The massacre has revived the national gun control debate and led to proposals for universal background checks on gun buyers and bans on assault weapons and high-capacity ammunition magazines. It also prompted reviews of school security and mental health care and led to proposed legislation in Connecticut that would forbid arcades and other establishments from allowing children under 18 to play point-and-shoot video games.

Prosecutor Stephen Sedensky III successfully argued in December to keep search warrant affidavits and applications related to Lanza's house and the car he drove to the school sealed for 90 days, saying disclosure would jeopardize an ongoing investigation. He said at the time no arrests were anticipated but had not been ruled out.

News media advocates say the records should be unsealed, arguing the public has a right to see such records, which include what was found in the house and car. They say records may be sealed only when an investigation would be hurt by disclosure and that the sealing does not appear justified since no prosecution is likely.