An investigation examining hundreds of leaked IRS forms offers a glimpse into the often-hidden challenges the U.S. tax agency faces in tackling the favorite new global investment vehicles of the ultrawealthy. That review and interviews with more than a dozen former tax officials show an agency struggling with not only a shortage of experienced agents, but also with an entire regime of federal rules — some of those created by the IRS itself — that have enabled investors seeking secrecy to run circles around the agency.

Accountants and lawyers who prepare these investors’ tax returns have eagerly exploited weak rules and years of lax enforcement to heap layers of secrecy between their rich clients and the IRS agents attempting to audit them.