Trump's tax returns aren't the only crucial records prosecutors will get

In addition to the tax returns, Mr. Trump’s accountants, Mazars USA, must also produce business records on which those returns are based and communications with the Trump Organization. Such material could provide important context and background to decisions that Mr. Trump or his accountants made when preparing to file taxes.

John D. Fort, a former chief of the I.R.S. criminal investigation division, said tax returns were a useful tool for uncovering leads, but could only be fully understood with additional financial information obtained elsewhere.

“It’s a very key personal financial document, but it’s just one piece of the puzzle,” said Mr. Fort, a C.P.A. and the director of investigations with Kostelanetz & Fink in Washington. “What you find in the return will need to be followed up on with interviews and subpoenas.”