(CNN) Sen. Bernie Sanders has pledged that he will release his tax returns from the last 10 years by Monday.

His personal financial information is under more scrutiny now that he is the front-runner in the 2020 Democratic presidential primary. His taxes will also reveal any information about his wife's income, especially during her tenure as president of now-defunct Burlington College. Questions persist around Jane Sanders' controversial leadership of Burlington . A federal investigation into a land deal involving the university was closed with no charges filed.

The senator released only a single year of tax returns during his 2016 presidential bid, from 2014, while his opponent Hillary Clinton released eight years of returns. Sanders also delayed filing his presidential campaign personal financial report with the Federal Election Commission, asking for two extensions, and ultimately never filed after losing the nomination to Clinton.

Since then, Sanders has become one of the 1%, the group of wealthy Americans he rails against in campaign stops, grossing an income of over $1 million. Sanders admitted he was a millionaire to the New York Times on Tuesday, saying "I wrote a best-selling book. If you write a best-selling book, you can be a millionaire, too."

The Sanders campaign did not answer questions about his income for this story.

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