I had posted this earlier in another thread, but I think this letter from Carl Levin to IRS is so significant, it needs a thread of its own.
In Obama’s 2009 press conference on FATCA, he credited Carl Levin for being one of the masterminds on FATCA for combating  offshore tax evasion.
In 2012, Carl Levin wrote a letter to IRS, demanding (page 11)

Although FATCA is structured to address offshore tax abuse, offshore account information has significance far beyond the tax context, affecting cases involving money laundering, drug trafficking, terrorist financing, acts of corruption, financial fraud, and many other legal violations and crimes. Given the importance of offshore account disclosures, FATCA guidance and implementing rule should create account FATCA forms that are not designated as tax return information but, like FBARs, may be provided to law enforcement, regulatory, and national security communities upon request. FFIs are not, after all, U.S. taxpayers, and will not be supplying tax information on behalf of their U.S. clients; they will instead be providing information about accounts opened by U.S. persons. The U.S. Supreme Court has long held that bank account information is not inherently confidential but is subject to inspection by law enforcement and others in appropriate circumstances. Foreign account information is too important to a wide range of civil and criminal law enforcement and national security efforts to be designated as tax return information bound by Section 6103’s severe restrictions on access.

Why bother with a warrant or surveillance when you can simply declare someone a “US person” and FATCA them?!?