Adam Lanza circa 2005 Twiiter/ABC Adam Lanza, 20, killed 27 people — including 20 kids at Sandy Hook Elementary School in Newtown, Conn. — before shooting himself, according to police.

His 24-year-old brother Ryan, who was initially named as the shooter, told friends that he thinks his developmentally disabled brother may have committed the crime.

Ryan told investigators he last saw Adam in 2010 and that Adam is autistic, a source told The New York Post.

Neighbors described Adam Lanza to ABC as “odd” and displaying characteristics associated with obsessive-compulsive disorder. He did make the honor roll in ninth grade.

Adam Lanza entered the school wearing all black, a mask and a bullet-proof vest while carrying Glock and Sig Sauer 9mm handgun, according to police. He was also carrying his older brother's ID.

A .223 Bushmaster rifle was found in his car. He died from a self-inflicted gunshot wound in the kindergarten classroom after firing dozens of rounds in two rooms.

Nancy Lanza, Adam's mother and a kindergarten teacher at the school, was found dead at her residence in Newtown.

The guns used in the shooting were legally purchased and registered to Nancy Lanza, Matthew Keys of Reuters reports.

The Post reported that Adam Lanza "had a dispute" with his mother.

SEE ALSO: 27 DEAD IN SCHOOL SHOOTING; SHOOTER KILLED MOTHER AND HIMSELF