Suspect Omar Mir Seddique Mateen was interviewed by the FBI twice in 2013 and again in 2014 for alleged "terrorist ties".
He pledged allegiance to the Islamic State of Iraq and the Levant (ISIL also known as ISIS) group on Sunday shortly before the killings.
Former FBI officer and a senior vice president of the Soufan Group, Martin Reardon, explains what investigators will be looking at now.
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