WASHINGTON -- Presidential candidate Mitt Romney's tax returns may be more incomplete in their public release than was previously thought. Filings related to Romney's individual retirement account, already subject to press scrutiny, indicate that the presumptive GOP nominee should have filed a form accounting for unrelated business income taxes.

The unrelated business income tax is set up for tax-exempts like nonprofits and IRAs that engage in commercial activity. Not filing the form, known as a 990-T, may indicate that Romney's IRA funds are held by an offshore account in order to shield them from taxation.

Romney campaign spokesperson Andrea Saul did not return multiple requests for the 990-T form, nor did she answer repeated questions related to the matter.

From what Romney has disclosed, there is mounting circumstantial evidence that the IRA may hold offshore investments through what are known as "blocker corporations," which help him avoid paying taxes. Senator Max Baucus (D-Mont.) told The New York Times, "From what I have read about Governor Romney's tax returns, I think it raises very serious questions."