Washington (CNN) Sen. Bernie Sanders said Tuesday he's a millionaire -- which will be confirmed when he releases 10 years of tax returns in the coming days.

The Democratic presidential candidate made no apologies for his growing nest egg; in an interview with The New York Times , he said, "I wrote a best-selling book. If you write a best-selling book, you can be a millionaire, too."

2020 Democratic presidential candidate U.S. Sen. Bernie Sanders (I-VT), R, speaks at a campaign rally in Grand Park on March 23, 2019 in Los Angeles, California. (Mario Tama/Getty Images)

The details of how Sanders became a millionaire will come to light when he releases his tax returns. Campaign manager Faiz Shakir told CNN Sanders' returns will be made public "on or before" Monday, the tax filing deadline for 2018.

The Vermont independent has repeatedly promised to release the information "soon." On Tuesday, Shakir said, "It will be done before the tax filing date is over. To my perspective, that is soon. I appreciate that there's been a lot of concerns that have been raised by a number of people but this will soon be over."

The release will likely quiet a growing din of concern over the release of Sanders' tax returns, which are expected to reflect an increase to his income through the sales of his book. But the unveiling will also open up Sanders, who released a single year of returns during the last campaign, to increased scrutiny -- the kind befitting his frontrunner status in this increasingly competitive and crowded Democratic primary field.

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