Donald Trump drew a red line for Special Counsel Robert Mueller when it came to looking at Trump’s financial records. But if there’s a red line out there, Mueller doesn’t see it.

The former FBI director leading the probe into the Trump campaign’s possible ties to Russia is taking a page from the playbook federal prosecutors have used for decades in criminal investigations, from white-collar fraud to mob racketeering: Follow the money. Start small and work up. See who will “flip” and testify against higher-ups by pursuing charges such as tax evasion, money laundering, conspiracy and obstruction of justice.

The finances of Donald Trump and those around him have become a critical part of the picture as the special counsel draws together his case.

The Virginia home of former Trump campaign chairman Paul Manafort was subject to a pre-dawn, no-knock raid by FBI agents seeking information likely related to the $12 million in under-the-table funds Manafort is accused of getting for his work for pro-Russian factors in Ukraine, or the mysterious set of collateral-free loans that have allowed him to add two more homes to a portfolio that includes a condo in Trump Tower. Mueller has extended this push on Manafort by pulling in Manafort’s son-in-law, whose own finances are entangled in a series of real estate deals and off-shore accounts.

But it’s not just Manafort who has been a target of Mueller’s growing team of powerhouse lawyers and investigators. Donald Trump Jr. is reported to be a focus of the special counsel’s efforts, with particular attention to the circumstances surrounding the meeting that Trump Jr. hosted at Trump Tower in June of 2016, where the Trump campaign was offered material attacking Hillary Clinton by representatives of the Russian government.

And those are just the start.