Mueller’s involvement now does suggest that the current focus relates to Manafort’s role in the Trump campaign. But we don’t know exactly how, if at all, any alleged crimes by Manafort relate to his work in that role. And we don’t know whether any other individuals involved in the campaign are potentially implicated.
We also don’t know what evidence was obtained as a result of the surveillance. The fact that warrants were issued does not mean any evidence of criminal conduct was actually found.
The other import of this news involves the possible implications if Manafort is charged. The New York Times reported Monday that when Manafort’s home was searched in July, investigators told him he should expect to be indicted. Even if Mueller were to indict Manafort for crimes not directly related to the Trump campaign, it would be a significant development. A typical white-collar investigation often proceeds by building cases against lower-level participants in a scheme — the little fish — and then persuading them to cooperate in the investigation of the bigger fish. Trump and his associates therefore may have reason to be concerned about what Manafort could tell investigators if he were indicted and chose to cooperate.