An internal actor seems more likely to me, but who knows?
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.
In the Ross Ulbright case the FBI got the bitcoin and then the two FBI agents stole it.
In the FTX case the crypto was taken from the exchange in the days before it declared bankruptcy so the FBI was not invovled.
Not sure where you read that they were. Can you provide a source for that?