If memory serves me correctly, they wanted to probe this guy further, perhaps even arrest him for his contacts with radical Imams.
This might clear it up a bit.
http://hotair.com/archives/2016/06/14/why-the-fbi-dropped-the-investigation-of-omar-mateen/
I get that the FBI didn't have enough to charge his ass in 2013/2014.
I understand that. Bad guys don't always make confessions to an undercover in a bar. It's very difficult to stop a crime before it happens AND bust the bad guy. I get that.
BUT I don't get why the FBI didn't do anything when they were told - 3 weeks ago by a store owner - that this dude was trying to buy bulk ammo and body armor. The store owner thought he was shady, so he REFUSED the sale and CALLED THE FBI.
If the feds have a guy on their radar for a year - and don't have enough - then they get new info, and don't act on it - that's doing a terrible job. I bet those agents had time to tweet, had time to take shits, had times to jerk off... but in those 21 days, couldn't interview the dude again, watch him, etc.
If he's planning an attack, and the feds stop in... they'll know pretty quick if it smells funny, they can watch him more, maybe stop it.