Garrett M. Graff, Watergate and FBI Historian:
1) The idea the FBI launched a raid on a former president would have been approved and monitored at the highest level of the Justice Department; hard to even imagine how high the bar of probable cause must've been for the Bureau to initiate such a politically sensitive search.
2) A search warrant means an independent federal judge ALSO signed off on the probable cause and, independently, believes evidence there was likely a crime committed AND that more evidence would be found at Mar-a-Lago. That's huge too.
3) The fact the search apparently didn't leak until basically when word came from Donald Trump himself shows the FBI and the Justice Department conducted this search by the book and a high degree of integrity. No leaks? Impressive. Surely only a small team knew inside DOJ.
4) Taken together, this is one of the most significant, sensitive, and politically explosive actions the US Justice Department and FBI has ever taken—one of a tiny handful of times it's ever investigated a president. Bottom line:
The FBI & DOJ must've known they had the goods.
High level and probably multiple cooperating inside witnesses to tell them exactly where and what to find. Especially if it only lasted an hour or so, and they specifically checked the safe.
And let's not forget this about the current FBI director: