The FBI knew Colorado grocery store shooter Ahmad Alissa before he executed his attack.
The FBI took no action.
https://www.nytimes.com/live/2021/03/23/us/boulder-colorado-shooting
The FBI knew in advance of the Pulse Nightclub shooter (Omar Mateen) and were tipped off by the local sheriff.
The FBI knew in advance of the San Bernardino Terrorists (Tashfeen Malik).
The FBI knew in advance of the Boston Marathon Bombers (the Tsarnaev brothers) tipped off by Russians.
The FBI knew in advance of the Parkland High School shooter (Nikolas Cruz).