It's Paul Manafort. Reports have been circulating for days that Robert Mueller was about to take his first person into custody as part of his wide-ranging investigation, and the New York Times and CNN report that Manafort, Trump's former campaign manager, is the one. In fact, the AP reports that Manafort surrendered to federal authorities Monday morning, along with business associate Rick Gates. The charges remain unclear, and they may not be directly related to the Trump campaign—the Wall Street Journal reports that the alleged violations include tax fraud.
Manafort had been under scrutiny over alleged tax law violations and for his foreign lobbying, particularly in Ukraine. Spokespersons for the various parties weren't shedding much light, but details should emerge soon. Mueller has been looking into possible collusion between the Trump campaign and Russia, as well as into whether President Trump obstructed justice when he fired FBI chief James Comey. (The FBI has been investigating suspicious wire transfers linked to Manafort involving about $3 million.)