Five years after the Newtown massacre, stunning warning signs revealed in FBI report

It's been nearly five years since the horrific Sandy Hook Elementary massacre, and new documents released by the FBI show that there were stunning warning signs about the shooter, Adam Lanza.

What were the warning signs?

A resident told police that Lanza had threatened to kill his mother and students before he went on his killing spree. She said that she overheard the 20-year-old say that he "planned to kill his mother and children at Sandy Hook in Newtown, Connecticut."

She said that he "had an assault weapon and that she was scared of him."

According to the report, she called the Newtown police, but they told her that since Lanza's mother owned the guns legally, there was nothing they could do and that she should try contacting the state police.

Another source said that Lanza's mother was concerned about him being a "shut in" who had not left his room in three months.

Another source said that Lanza rejected the medication he was provided for his Asperger's syndrome condition, a diagnosis he also rejected.

"Nancy [Lanza] would take care of all of Adam's needs," the person reportedly said. "However, she never cleaned his room, nor was allowed in his room. Adam's room was his personal space that no one else was allowed into."

Where is this information from?

The FBI released 1,500 pages from the investigation Tuesday into the shooting that stunned the nation. Some of the information was heavily redacted, according to CBS News. Lanza killed 20 students and six adults in the Sandy Hook Elementary School attack on Dec. 14, 2012. The shooting spree ended when he shot himself.