The mystery of what could be in Mitt Romney's still unreleased-tax returns deepened once more this weekend, when the Republican presidential hopeful let slip that he has been audited "from time to time" by the IRS.

Speaking to ABC's David Muir during a sit-down in Israel Sunday, Romney said that he couldn't say whether he has ever paid less than a 13.9% tax rate, but that he has "paid all the taxes required by law."

Via the ABC News transcript:

Muir: You are here on what some have termed your world audition and democrats, not surprisingly, continue to hammer you back home on taxes, you remain firm two years and two years only. So from what you have released and from what we have seen we know that there was one year when you paid about 13.9% tax rate. Can we clear this up by asking a simple yes or no question? Was there ever any year when you paid lower than 13.9%?

Romney: I haven't calculated that. I'm happy to go back and look but my view is I've paid all the taxes required by law. From time to time I've been audited as happens I think to other citizens as well and the accounting firm which prepares y taxes has done a very thorough and complete job pay taxes as legally due. I don't pay more than are legally due and frankly if I had paid more than are legally due I don't think I'd be qualified to become president. I'd think people would want me to follow the law and pay only what the tax code requires.

Muir: You said you would go back and look, would you look for us?

Romney: I haven't looked at the tax rate paid year by year. I know that I pay a very substantial amount of taxes and every year since the beginning of my career so far as I can recall.

Although the remarks have been largely lost amid news from Romney's overseas trip, they raise even more questions about the Republican presidential candidate's murky financial records. How many times has Romney been audited by the IRS? In what years? And what did the audits find?

We emailed these questions to the Romney campaign. Here's the response, from campaign spokesperson Ryan Williams:

“Mitt Romney has been scrupulous about observing the requirements of the tax code. Mitt Romney is in full compliance with U.S. law and he has paid 100 percent of what he has owed.”

Watch the interview below:

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h/t First Read