The FBI et al are very good at surveillance. I doubt anyone close enough to SBF to be part of a criminal conspiracy would be unknown to the FBI or not under surveillance at this point.
Being _known_ by the FBI doesn’t stop crime or terror attacks. More often than not when something big happens the FBI was already tipped off, an investigation started and then somehow someway the crime/attack/incident still happens.
If someone is willing to steal from their customers and injure innocent, random public, then it seems like a small character epsilon to also steal from their employer and its investors.