The crime is done and he killer is dead. Why do they need it?
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He had accomplices. Or people who knew what he was going to do. This idiot had a huge media footprint. His FB page, his pro-Antifa postings, his texts - everything!
People knew. Perhaps even his family.
And what did the FBI do before Smart phones? Interview. Whatever a phone has, it’s inferior to what can be gained by interviewing.