I don’t know whether the IRS proactively shares information with law enforcement, though I wouldn’t be surprised, but I do know with absolute certainty that the IRS regularly works with law enforcement on many issues that don’t necessarily relate to tax evasion. Money laundering being chief among them.

There’s a bi-weekly process called “314(a) Information Sharing” that’s run by the Treasury Department under the USA PATRIOT Act. It requires financial institutions to pass all kinds of information on to the agencies in the Department - IRS, Secret Service, Customs, ATF, Homeland Security, etc. - who are looking into various matters. All of those agencies are under the same umbrella and they absolutely work together.

A good friend of mine is also a forensic accountant/investigator for the US Attorney’s Office so I know the IRS works closely with the DOJ though I’m not familiar with how that process works in practice.

It isn’t a huge secret: https://www.irs.gov/government-entities/federal-state-local-...

> IRC 6103(i)(1) provides that, pursuant to court order, return information may be shared with law enforcement agencies for investigation and prosecution of non-tax criminal laws.

They need a warrant, the IRS can’t proactively share tax info with law enforcement agencies for non-tax law reasons.