Mitt Romney escalated the pressure for Donald Trump to release his tax returns, doubling down on his belief that the presumptive Republican presidential nominee has something to hide.

"Your imagination can run wild," Romney said in an interview with CNN's Wolf Blitzer on Friday.

He continued: "He doesn't pay taxes. He doesn't have much income. He's receiving income from unsavory sources. He has ownership in enterprises that are perhaps associated with unsavory enterprises or nations or groups."

Romney, one of the most prominent Republican critics of Trump, said in a Facebook post last month that it's "disqualifying" for presidential candidates not to release their tax returns, and said that Trump's returns could contain a "bombshell of unusual size."

He is just one of a growing number of voices calling on Trump to make his returns public, a tradition observed by major-party presidential candidates since the early 1970s.

Trump has resisted the call so far, and Romney said that "there is no question" that Trump will never come around.

"There's something in those taxes that's even worse than shooting someone on Fifth Avenue," Romney said, referencing a joke Trump made during his primary campaign that he could shoot somebody in the middle of the street and not lose the backing of his supporters.

"He will follow one excuse after the other to say why he can't," he said. "And he'll never release them, because there's something in there that he feels is worse for him than shooting someone on Fifth Avenue."

Trump has cited an ongoing IRS audit as the reason why he has not made his tax returns public.

Watch a segment from the interview below: