Since the incidents, involving up to 130 U.S. personnel, have happened in Cuba, China, Russia, and elsewhere (including at least one possible incident inside the U.S.) it would have to be someone with the resources to carry out such attacks across the world (and the CIA director said as much if you listen to the interview). The most likely culprit being eyed by Pentagon officials is Russia (as previously reported[1]), but the CIA director chose not to name names during this latest interview even when directly asked if it was Russia.
[1] https://www.nytimes.com/2021/05/12/us/politics/biden-cia-bra...