WASHINGTON — The opaque finances of uber-wealthy Republican presidential challenger Mitt Romney came under the microscope again Thursday after a fresh document dump by a New York-based blog site.

Gawker published more than 950 pages of internal audits and financial statements from 21 funds, almost all of them affiliated to Bain Capital, the private equity firm Romney co-founded on his way to amassing an estimated $250 million fortune.

The dump shed fresh light on the labyrinthine investment portfolio that has allowed Romney to keep his tax rate at roughly 13 percent, well below that of most working-class Americans who earn far less.

However experts agreed there was nothing especially damning in the files -- certainly no smoking gun for political opponents seeking evidence of wrongdoing or illegality.

"It appears that folks looking for a bombshell are going to be disappointed," wrote Joe Weisenthal of Business Insider.

But the documents did pour more unwanted scrutiny -- as far as Romney was concerned -- on a fortune that is largely squirreled away in tax havens like Bermuda, the Cayman Islands and Switzerland.

"They reveal the mind-numbing, maze-like, and deeply opaque complexity with which Romney has handled his wealth, the exotic tax-avoidance schemes available only to the preposterously wealthy," Gawker journalist John Cook wrote.

Attacking Romney over his wealth is viewed as fair game by Democrats, especially as the Republican challenger touted his record at Bain as evidence that he has the business acumen to turn around the flagging US economy.

President Barack Obama's campaign says Romney's financial secrecy means he has plenty to hide and has linked his wealth with a string of gaffes to portray the Republican challenger as out-of-touch with ordinary Americans.

Republicans have hit back by accusing Democrats of attacking success. Romney says he has done nothing illegal and points out that most of his money is managed through a blind trust.

The Republican challenger has defied mounting pressure from the Obama campaign to release more than two years of tax returns.

Romney told Parade Magazine that part of the reason he does not want to release more tax returns is that he believes his donations to the Mormon Church should remain a private matter, reports said.

"Our church doesn't publish how much people have given," Romney reportedly told the magazine in an interview for a forthcoming edition.

"This is done entirely privately. One of the downsides of releasing one's financial information is that this is now all public, but we had never intended our contributions to be known. It's a very personal thing between ourselves and our commitment to our God and to our church."

Romney's 2010 tax returns and an estimate of his 2011 returns reveal that over those two years he and wife Ann gave more than $4 million to the Mormon Church, 10 percent of roughly $40 million of disclosed income.

Mormons are expected to tithe at least 10 percent of their annual income to the church.

Mormonism, which originated in the 1820s in New York state as a fusion of Christian theology and other teachings, is still viewed with suspicion by many in the United States.

Romney would become the first Mormon to be elected president if he defeats Obama in the November 6 election.