"They" only could be, by plot/tied to Egghead, Luffy, Vegapunk:
- Blackbeard Pirates.
- Revolutionary Army.
- Cross Guild.
- Unknown party with unknown intentions (underworld for weapons to start a war/the war...).
If "they" destroyed a Marine ship, then they are not related to World Government or Marines (including CP0, Holy Knights...).
So, potentially I would see Blackbeard Pirates as the most obvious reason because:
- York (she wanted to be a Celestial Dragon OR a pirate instead... So maybe she made a deal with the Blackbeard Pirates before as a plan B).
- Caribou (Blackbeard is an option to be Caribou's mysterious boss).
- We saw a ship from Blackbeard Pirates approaching to Egghead and time has passed since that and we still do not know anything about them, even the Marines/World Government didn't talk about them.