Shouldn’t this be pretty simple? Get information on who was logging into the account and from where from the provider?
I'm not sure of the legal technicalities but I think they need a warrant which means they'd need evidence of malfeasance to get one which means somebody is a rat.
This whole Manafort episode is bad ju-ju for the heavy hitters in the Hapsburg group. Their image is built on being statesmen & sages, not hired guns. Yeh, sure, the money is good but the publicity is very bad. This may not have been the first & only sleazy deal they ever worked, either. It may well be causing them trouble with European authorities.
They're all kinda tied together thru the money trail, too. So having anything to do with Manafort is poison, at least for some, but they can't trust the others to be that smart, either. So of course they'll rat out Manafort in a heartbeat to neutralize that threat. He can't fuck it up any more than he already has if he's in jail.
It's also the kind of thing the authorities truly appreciate, as well.