An FBI compound located in Dumont, Ohio.
After his return to Ohio, John risks seeing Sarah but is suddenly surrounded by FBI who take him and Sam to their facility in Dumont. John is interrogated by Detective Will Murphy. The facility is infiltrated by Six and attacked by the Mogadorians simultaneously. Six frees John and Sam and they manage to escape (The Power of Six).
When Mark James discovers from Sarah that John is back in Paradise, his father sees her messages and calls the FBI. Mark listens to his dad's old police scanners and hears talk of the Dumont Facility which he discovers is a large, strictly off limits, FBI compound (Return to Paradise).
This facility does not appear to be an FBI facility that the Mogadorians had control of since they had to force their way in to get to John.