As Mitt Romney’s refusal to release further tax returns becomes a central issue in the presidential race, a related question first raised in January remains stubbornly unanswered: How did Romney build such a massive individual retirement account?

The size of the GOP presidential candidate's IRA is "very unusual," said Rebecca Wilkins, senior counsel for federal tax policy at the nonprofit advocacy group Citizens for Tax Justice. "The Romney campaign has not been that forthcoming."

According to Romney’s disclosure documents, the candidate has between $20.7 million and $101.6 million parked tax-free in his IRA -- a significant proportion of his total wealth, as noted by Mark Maremont in the Wall Street Journal.

It’s a whopping sum by most standards; according to the nonprofit Employee Benefit Research Institute, the average IRA held $67,438 in 2010.

Since that figure was first reported, questions about Romney's IRA have continued to dog his campaign; a recent column by William Cohan in Bloomberg View and a thorough investigation of Romney's finances in Vanity Fair both cast a skeptical eye on the account.

There are limits to the amount of money individuals can contribute to their IRAs. Before Romney was elected Massachusetts governor, federal law capped annual pre-tax IRA contributions at $2,000 and annual 401(k) retirement contributions at $30,000 with a company match as the Journal notes. Cohan reported that Bain used what’s known as a SEP-IRA during Romney's time as head of Bain, which has a slightly higher yearly maximum contribution of $30,000.

Given these contribution limits, experts are scratching their heads about the rapid growth of the account in the decade since Romney officially left the private equity firm Bain Capital.

“It’s extremely unusual,” said George Yin, a professor of law and taxation at the University of Virginia and former chief of staff of the U.S. Congress Joint Committee on Taxation. “The IRA, since its inception, has had very clear limits on the amount that can be contributed each year. And it's way less than $100 million.”

The Romney campaign has offered little explanation; Andrea Saul, a Romney campaign spokesperson, did not respond to a request seeking comment for this story. In the Journal report, a Romney aide said the IRA accumulated 'through annual contributions, rollovers of sums in other retirement plans, and successful investments."

But there are a few specific theories that attempt to explain the account's growth.

The first is that Romney acquired stock from his Bain investments -- specifically high-risk, high-reward shares that had a low initial value (and thus didn’t violate the contribution limits) but paid off when the companies became more profitable. If he filled his IRA with these stocks, that could account for some of the exponential growth, as Cohan notes.

“Even though the contribution limits are small, if the stock growth is tremendous, you could possibly get up to something close to [$100 million],” said Yin, adding “though it still seems amazing to me.”

If that's the explanation, than Romney’s partners at Bain would likely have seen similar returns, and might even have similar amounts in their IRAs. “Why aren’t all the Bain partners coming forward and saying ‘yeah, we all have 100 million IRA’s too?” Cohan asked in an interview with The Huffington Post.

Another possible explanation, noted by Felix Salmon in a recent Reuters blog post, is that the growth is not from shares in Bain investments Romney shoved into his IRA, but rather shares in the private equity firm itself of which Romney was the sole shareholder until he left in 2002.

A sophisticated but legal method of shielding income from taxes, which involves investing an IRA in what is known as “blocker corporations,” as pointed out in Vanity Fair, may also have played a role.

By law, most investment income that flows into an IRA is tax-free with the notable exception of income that comes from running a business, said Ed Slott of Ed Slott and Co., a tax expert who specializes in IRAs. That income is subject to a special tax known as the Unrelated Business Income Tax.

But one way to avoid that tax is to set up a blocker corporation, an intermediary company that on paper own the companies. That allows income received from the investments to be counted as an investment dividend and thus avoids taxes.

“It’s an extraordinary maneuver,” said Slott.

If Romney's IRA does indeed use blocker corporations, it means “he created this buffer, this intermediary entity to turn what would be taxable income into tax-deferred income in the IRA,” Slott said. "That’s not what most people [are able to] do.”

There’s another big question that leaves experts scratching their heads: Why does Romney have all that money in a retirement account to begin with?

While IRAs are tax exempt now, by law Romney must begin withdrawing cash by age 70 and a half. When he does, “the withdrawal receives no special tax breaks. It is subject to ordinary income tax rates, which right now are as high as 35 percent,” said Slott.

“I think it is unusual to earn these profits in IRA accounts,” for precisely this reason, said Richard Bronstein, a tax lawyer at Paul, Weiss, Rifkind, Wharton & Garrison. “It’s more common to earn this in taxable accounts, because capital gains rates are not really high.”

It the money came from his income as a private equity chief, it would be taxed at the current carried interest rate of 15 percent, rather than the income rate of up to 35 percent.

What’s more, said Bronstein, “if he has $100 million in his IRA, it’s not very accessible. You accumulate a fortune in a tax-exempt account, but you can’t use it to buy a boat or an elevator for your car.”

So far, Romney has said little publicly about his retirement account, leaving experts and the public with little information beyond speculation. “Nobody really knows what the explanation is,” said Slott. “Everybody’s guessing.”

One thing is clear. While most Americans have the ability to open an IRA, very few have the opportunity to engage in the sort of scrupulous tax planning Romney has shown.

“Romney is an aggressive tax planner,” said Wilkins.”He’s not just collecting his paycheck and dividends from Bain, he’s planning his affairs in the most tax advantageous [way possible].”

Still, Wilkins added, “I used to be a C.P.A. in private practice. It was not unusual for someone to roll over a $3 million IRA. But $20 or $100 million? It’s unfathomable.”