Melania Trump
First Lady Melania Trump, pictured with President Donald Trump in Washington, D.C., April 6, 2017, was criticized by some for wearing a sleeveless dress to the Wounded Warrior Project Soldier Ride event at the White House. Getty Images

When asked whether President Donald Trump will release his tax returns, White House Press Secretary Sean Spicer said Monday that Trump cannot do so as he is under audit by the Internal Revenue Service (IRS). If Trump continues to refuse to release his tax returns in office, he will reportedly become the first American president in 40 years to do so.

In the press briefing, Spicer said that Trump had made it clear during the campaign last year about the status of his income tax returns. Trump has used the same explanation about him being under audit by the IRS since early last year, according to reports.

Read: Trump’s 2005 1040 Tax Filing Summary Released On Air

Trump is not the first president whose tax returns have created controversy. In the decadeslong history of presidential tax returns, there have been others as well whose tax returns have raised questions.

In 1973, when the Watergate scandal was at its peak, tax experts demanded for an audit of the then-President Richard Nixon. The IRS refused, but one of the agency's employees leaked information showing that despite having an income of more than $200,000, Nixon paid only $792.81 in federal income taxes in 1970 and $878.03 in 1971.

Nixon also took a huge number of deductions, which included $570,000 for the gift of his vice-presidential papers to the National Archives. He apparently asked his aide to show that transaction as a backdated entry to the previous year. Nixon eventually had to his show his tax returns going back to 1969, and was later hit with a tax bill of $471,431 plus interest.

“People have got to know whether or not their president is a crook,” Nixon told reporters in November 1973, amid the scandal over his taxes. “Well, I am not a crook.” He resigned less than a year later.

Former President Ronald Reagan’s 1981 tax return showed that he paid taxes at an effective rate of 40 percent. However, Reagan took advantage of the signature tax cuts that he passed when he was in office. In 1987, he gave only 25 percent of his income to the IRS. According to 1987's tax returns, the former president's income that year included not only his presidential salary, but also his income from his movie career and autobiography.

In 1994, the then-President Bill Clinton and his wife, Hillary Clinton, released their tax returns for several years to silence all those questioning the couple's Whitewater Development Corporation, which was formed in 1978. The real estate venture failed but some claimed it involved illegal dealings by the couple. Three separate inquiries were held but no evidence were found against the Clintons.

Then-Senator Barack Obama's tax returns in 2008 reflected a huge spike in charitable donations during his presidential campaign. The New York Times also reported at the time how Obama's income rose drastically from book royalties. Some of Obama's largest donations went to the Trinity United Church of Christ. In 2005 and 2006, the church reportedly received $27,500 from Barack Obama and his wife, Michelle.