A friend of Sandy Hook Gunman Adam Lanza's mother said the gunman may have carried out the shooting spree as an act of revenge, after suffering from years of bullying at Sandy Hook Elementary School.

Marvin LaFontaine said Lanza, who killed 26 people, had been fostering resentment toward the school for years, according to Mail Online.

"I think Adam felt betrayed by the school and this was his act of revenge," LaFontaine told the New York Daily News.

His mother, Nancy Lanza was angry with the school for not protecting her son, and, according to LaFontaine, would sometimes sit in his class to ensure his safety.

"Adam didn't like her showing up. She would sometimes sit in the back of the class and make sure no one would touch him," he said, according to the Mail Online report.

LaFontaine was also Lanza's Cub Scout leader when he was a young boy and said he was told by the killer's mother that he was picked on at school. He said, "Adam was an easy target. He was quiet and he would never fight back."

On December 14, the 20-year-old shot his mother. He then aimed 155 bullets at innocent children and educators during a five-minute, bloody rampage, ending with 20 dead school children and six dead staff members before Lanza shot and killed himself.