Nicholas Cruz, 19, is accused of attacking a school in Florida on Wednesday night, killing 17 people.
He is said to be in possession of at least one AR-15 weapon, a gas mask, smoke grenades and ammunition.
Activate the fire alarm to get students out of the classroom to open fire.
He was expelled from school
Victoria Olvera, 17, said Cruz was expelled from school last year and caused a quarrel with his ex-girlfriend's new boyfriend. He said it was abusive for his girlfriend.
Dakota Mutchler, who was a friend of his, said he had not seen him for more than a year after his expulsion, and before the incident he had gradually begun to behave strangely.
The other students reported that his behavior had changed resulting in him losing company and friendships.
Very annoying things on his online profile
Police say they are beginning to "analyze" the suspect's online profiles.
The Broward County Sheriff described some things that had already been found as "very, very annoying."
Shortly after the suspect was arrested, YouTuber Ben Bennight referred to a comment he received in one of his videos a few months ago from the suspect, in which he said that he wanted to be a "professional school sniper". The FBI has not confirmed that the account was linked to Cruz.
His stepmother died in November
Cruz and his brother Zachary were adopted by Lynda Cruz and her husband when they moved from Long Island to Broward County.
His stepfather, Roger, died of a heart attack several years ago. His mother died of pneumonia last November at the age of 68.
He went to live with a family friend, but was unhappy there and moved with a friend and family to northwestern Broward. They knew he had a gun and persuaded him to keep it in a locked locker, but he had the key.
He was calm at the time of his arrest
Cruz was taken into custody without resistance, about an hour after the incident.