I think they'd want to look at email trails (and local copies of such e.g. Outlook data files) as much as looking for the 'big datasets' that Cambridge Analytica claims are now deleted. It's more likely, maybe, that internal emails from the time of the US presidential elections are still archived, and those would shed a light on the company's true conduct at the time.

I don't know if they're allowed to seize actual computers/hard drives (let's hope they have on-disk encryption), but I imagine that would be very valuable too.