Federal authorities have opened an investigation into two Wisconsin-based corporations founded and run by Mark Block, a veteran political operative who ran Herman Cain's unsuccessful presidential campaign.

"They are very interested in Mark and these groups," said a source familiar with the probe. "It is not my sense, right now, that Cain is a target."

In recent weeks, FBI agents have been talking to donors and other individuals connected with Prosperity USA and Wisconsin Prosperity Network. Block and Linda Hansen, Cain's deputy chief of staff, were the two primary people running those groups.

No Quarterreported last year that Prosperity USA helped Cain get his fledgling campaign off the ground by originally footing the bill for such items as iPads, chartered flights and travel to Iowa and Las Vegas, according to internal records.

Expenses totaling nearly $40,000 are listed in Prosperity USA's internal documents as "due from FOH," a reference to Friends of Herman Cain, the name of his campaign committee.

A number of election law experts have said these payments for campaign events appeared to cross the line.

Prosperity USA also borrowed as much as $150,000 from two unnamed individuals and then gave the bulk of those funds to the Con gress of Racial Equality , a conservative civil rights group, in January 2011. Shortly after that payment was made, Cain - who had just entered the presidential race - was a featured speaker at the group's annual Martin Luther King Jr. Holiday Celebration dinner.

Sources said the loans, which don't appear to have been repaid, were obtained under questionable circumstances.

In addition, it does not appear that either organization was granted tax-exempt status, even though contributors were told they could write off their donations. Both groups received substantial support from major conservatives in the state.

Block did not return texts, emails or calls asking for comment on Thursday. He is now working for Cain's Solutions Revolution, a group promoting the former candidate's 9-9-9 tax plan.

Assistant U.S. Attorney Richard Frohling, who is spearheading the probe, also declined to comment, refusing even to acknowledge that there is an investigation.

Last year, several liberal advocacy groups filed complaints with federal election and tax regulators urging them to investigate Block and the two Wisconsin corporations. The Cain campaign responded by saying it had retained two law firms to look into the allegations.

Cain, the former CEO of Godfather's Pizza, has never disclosed the results of the internal investigation. He dropped out of the race in early December after being pummeled by allegations of infidelity.

Block was asked last month at a public forum if the Federal Election Commission was investigating the Cain campaign and the questionable donations from Block's private organizations.

"Not to my knowledge," he said.

The question, interestingly, came from Milwaukee attorney Walt Kelly, who was at the center of the last major controversy involving Block.

In 1997, Block was the campaign manager for then-Supreme Court Justice Jon Wilcox in a hotly contested race against Kelly. After the race was over, state regulators accused Block of election-law violations, including coordinating a mass mailing with an outside group.

Block settled his part of the case by agreeing to stay out of Wisconsin politics for three years and to pay a $15,000 fine.

More recently, Block ran the state chapter of Americans for Prosperity, a nonprofit cofounded by the conservative Koch brothers that helped organize the tea party movement in Wisconsin and elsewhere.

It was through Americans for Prosperity that Block met Cain and encouraged him to run for national office. Block's role with the Cain campaign became a point of national interest late last year when the campaign released a bizarre online ad featuring the chain-smoking Wisconsin operative.

Records show Block was paid more than $182,000 in total compensation during his year running Cain's failed campaign. He pulled down $84,605 in the final three months of 2011, even though his candidate dropped out of the race in early December.

Hansen, Block's political and business sidekick, received a little less than $88,000 in total compensation in 2011. Like Block, she deposited her biggest paychecks during the final three months of the year, taking in $56,430 during that span.

It's not surprising that they were paid the most at the end of last year.

That's also when the campaign had the most money.

Riding a short-lived wave of publicity and popularity, Cain received $11.3 million in donations between Oct. 1 and Dec. 31. But the GOP candidate spent all of that, plus $500,000 more, over the same period.

Last month, FEC officials sent the Cain campaign a 24-page letter outlining apparent errors in its past filings.

For example, regulators said the campaign received dozens of contributions exceeding the federal limits, took in improper donations from corporations, refunded money to people who weren't listed as making contributions, and made basic math errors regarding Cain's loans to the campaign.

"This notice requests information essential to full public disclosure of your federal election campaign finances," wrote Michelle Grant, a senior campaign finance analyst at the FEC. "Failure to adequately respond by the response date noted above could result in an audit or enforcement action."

Block just beat the deadline set by Grant, submitting a lengthy amended statement last week. It's not clear if the new campaign report addresses all of the issues raised by regulators.

The source familiar with the investigation said it doesn't appear that the federal probe is connected to the issues being raised by the FEC.

"I don't believe that," the source said. "Remember that there are a lot of people with a lot of zeros behind their names who are really pissed off."

Along with his campaign work, Block spun off a handful of organizations from Americans for Prosperity, most of them incorporating "prosperity" in the name. Officials with Americans for Prosperity emphasize that these other groups were legally separate from their organization.

The largest group founded by Block was called Wisconsin Prosperity Network, which was supposed to be an umbrella organization that would spend more than $6 million a year underwriting other conservative groups in hopes of turning the state red.

In the 2008 incorporation papers, Block is listed as the president of Wisconsin Prosperity Network, which was set up as a tax-exempt nonprofit group. That means the charitable organization can't have direct political involvement. Hansen was the group's executive director.

Two years later, Block started Prosperity USA, with Hansen again handling the day-to-day operations.

Internal financial records show both organizations were operating in the black during the first half of 2010.

But that changed last year.

Balance sheets showed Wisconsin Prosperity Network was more than $62,000 in the hole by early February; Prosperity USA was in even worse shape, with its liabilities exceeding its assets by $110,000.

Most - and perhaps all - of the board members for both organizations have resigned over Block and Hansen's financial dealings, according to insiders.

Contact Daniel Bice at (414) 224-2135 or dbice@jrn.com. Follow him on Twitter @DanielBice.