The suspect’s identity was previously known to the F.B.I. because he was linked to another individual under investigation by the bureau, according to law enforcement officials. When he was a senior at Arvada West High School, Mr. Alissa was convicted in 2018 of misdemeanor assault against another student in a classroom, and told the police at the time that it was in retaliation for insults and ethnic taunts. Fellow students recall him as having a fierce temper that would flare in response to setbacks or slights. Mr. Alissa’s brother described him to the Daily Beast as mentally ill, paranoid and...