Special counsel Bob Mueller has teamed up with the IRS. According to sources familiar with his investigation into alleged Russian election interference, his probe has enlisted the help of agents from the IRS’ Criminal Investigations unit.

This unit—known as CI—is one of the federal government’s most tight-knit, specialized, and secretive investigative entities. Its 2,500 agents focus exclusively on financial crime, including tax evasion and money laundering. A former colleague of Mueller’s said he always liked working with IRS’ special agents, especially when he was a U.S. Attorney.

And it goes without saying that the IRS has access to Trump’s tax returns—documents that the president has long resisted releasing to the public.

Potential financial crimes are a central part of Mueller’s probe. One of his top deputies, Andy Weissmann, formerly helmed the Justice Department’s Enron probe and has extensive experience working with investigative agents from the IRS.

“From the agents, I know everyone has the utmost respect for both Mueller and Weissmann,” said Martin Sheil, a retired IRS Criminal Investigations agent.