Nonprofit organizations, including colleges and universities, make billions of dollars in income from activities unrelated to their core missions, but in a review of tax documents filed by 91 such groups, The Chronicle of Philanthropy found that roughly half paid little or nothing in federal taxes on the income.

In its review of the organizations’ most recent 990-T tax forms, which were made available to the public for the first time thanks to a change in federal law, the newspaper found that 46 — or 51 percent — listed zero or negative taxable income after taking deductions and making other calculations. Among the 45 groups that calculated that they owed tax on unrelated-business income, five accounted for three-quarters of the taxes paid. Those five included Stanford and Emory Universities.

Sen. Charles E. Grassley, of Iowa, the senior Republican on the Senate Finance Committee, said the information raised “a significant number of questions that need to be looked at more closely by charity boards, Congress, and the Treasury Department.”