It is at least reasonably possible, if not likely, that Mateen was recruited as a terrorist while in Arabia. Whether he received any terrorist training while there, in view of his known marksmanship skill (head shot on a Orlando SWAT trooper stopped by the military-style helmet), depends on the firearms training he may have as a security guard.
And at that this point we must consider whether the 2nd FBI investigation of Mateen was even aware that there had been a prior FBI investigation of him.