Donald Trump refused to condemn racist groups that would support his candidacy, a few days after former Ku Klux Klan member David Duke endorsed him.

"I have to look at the group," Trump told Jack Tapper on CNN's "State of the Union" Sunday. "If you would send me a list of the groups I would do research on them and I would disavow."

He said that he knew nothing about Duke, a nationally-known former grand wizard of the Ku Klux Klan who said that voting against Trump is a vote against white heritage.

But within minutes of that claim, watchers unearthed evidence Trump knew of Duke and his KKK affiliation. A 2000 New York Times article on Trump's consideration of a presidential run with the Reform Party, founded by Ross Perot, quoted the real estate mogul on Duke.

"The Reform Party now includes a Klansman, Mr. Duke, a neo-Nazi, Mr. Buchanan, and a communist, Ms. Fulani," he said in the article referring to other Reform Party supporters. They were conservative commentator Pat Buchanan and Lenora Fulani, who was an advocate of Marxist-Leninist politis, the article said.

"This is not company I wish to keep," Trump said.

When asked on State of the Union the Anti-Defamation League's call for him to condemn white supremacists that support his presidential campaign, Trump demurred.

"I don't know what you are talking about with white supremacy," Trump said.

Meanwhile, Trump continued to say that he couldn't release his tax returns because he is being audited. He added that he feels he is being "singled out" by the Internal Revenue Service because he has been audited for almost every year.

The IRS has said that there is nothing that prevents Trump from releasing his returns.

Trump's opponents, Sens. Marco Rubio and Ted Cruz, both released years of returns on Saturday, pushing for Trump to do the same.