Former Massachusetts Gov. Mitt Romney said Wednesday that there could be a potential "bombshell" in the back tax returns that Donald Trump has yet to release as he moves through the GOP primary.

Appearing on Fox News, the 2012 Republican nominee said he wants the Republican presidential candidates all to release their tax returns for the last "several years," before hinting that there could be something fishy as to why Trump hasn't released his yet.

"Frankly, I'd like to see a number of things from the candidates," Romney said, pointing to policy proposals before bringing up taxes. "I'd also like to see their back taxes. I'd like to see where they've filed their taxes in the last several years. I'm not talking about their taxes this year. I'm talking about the taxes that have already been filed with the IRS, and Donald Trump, Marco Rubio and Ted Cruz have not shown us their back taxes."

"This will give us a sense of whether these people are on the up and up, and whether they've been telling us things about themselves that are true or not," Romney continued before touching on Trump. "I think, frankly, we have good reason to believe that there's a bombshell in Donald Trump's taxes."

"I think there's something there," Romney told host Neil Cavuto. "Either he's not anywhere near as wealthy as he says he is or he hasn't been paying the kind of taxes we would expect him to pay, or perhaps he hasn't been giving money to the vets or the disabled like he's been telling us he's been doing."

"The reason I think there's a bombshell in there is because every time he's asked about his taxes, he dodges and delays and says. 'Oh well we're working on it.' Hey, we're not talking about the taxes that are coming through this year. Of course they're working on those, they won't be ready for months."

The defeated presidential nominee told Cavuto that the candidates all should have released them already, noting he released his in early January when he was competing for the nomination four years ago.

Romney also conceded that Trump currently has the easiest route to the Republican nomination and that time is running out for Rubio and Cruz to catch him.

"Well I think there's no question but that Donald Trump has the clearest path to become the Republican nominee," Romney said. "I think for the other people still in the race, their path is becoming a slimmer and slimmer opening, and they're having a difficult time communicating to their supporters just how they could become the nominee."

"It's not impossible. I think it's very difficult for any one of the non-Donald Trump contenders to be able to be a real threat unless perhaps the field narrow," Romney said. "The field narrowing would create an opening, but that doesn't seem to be happening.