This post has been updated.

The federal probe that resulted in a guilty plea this week by a former Iowa state senator who was secretly paid to endorse two Republican presidential candidates in the 2012 campaign is ongoing -- and could implicate the political operatives who were involved in routing payments to him, according to people familiar with the case.

Kent Sorenson, a one-time tea party star and Iowa legislator, caused a stir in late 2011 when he dramatically dropped his endorsement of Rep. Michele Bachmann of Minnesota to back former Rep. Ron Paul of Texas.

One focus by federal prosecutors now appears to be on people involved in Paul’s 2012 operation.

Paul's campaign committee doled out $80,000 in legal fees this spring, according to federal campaign finance reports -- nearly twice as much as it spent on legal costs in all of 2013.

David A. Warrington, who served as general counsel to Paul’s 2012 presidential campaign, did not return requests for comment.

On Wednesday, Sorenson admitted in federal court what he had long adamantly denied: that he took thousands of dollars in payments from the campaigns of Bachmann and Paul in exchange for his endorsement.

The former state senator pleaded guilty to one count of causing a federal campaign committee to falsely report its expenditures and one count of obstruction of justice for giving false testimony to a state independent counsel investigating the payments.

Two sealed documents were filed in court as part of the case, a possible indication that Sorenson is cooperating with federal prosecutors as part of a broader investigation.

Sorenson’s attorney, F. Montgomery Brown, declined to comment on whether the former state senator had such a deal. But he and others involved in the matter said the case is continuing.

“It’s my understanding that this an ongoing investigation,” Brown said.

Justice Department spokesman Peter Carr confirmed that the investigation is ongoing.

Jesse Benton, a Republican strategist who is married to Paul’s granddaughter, served as chairman of Paul’s 2012 campaign. He is now running Senate Minority Leader Mitch McConnell’s reelection campaign and is expected to play a significant role in a 2016 presidential bid by Sen. Rand Paul (R-Ky.).

It is unclear if Benton knew about the payments made to Sorenson, but emails published last year by the TheIowaRepublican.com and OpenSecrets.org show that Benton wrote to a Sorenson associate in the fall of 2011, asking if the then-state senator would be “joining our team.”

Neither Benton nor the McConnell campaign returned requests for comment.

A key point person from the Paul campaign who urged Sorenson to defect was Dimitri Kesari, then Paul’s deputy national campaign manager. At a December 2011 meeting at an Altoona restaurant, he gave Sorenson’s wife a $25,000 check to secure the state senator’s support. The check was never cashed.

Jesse Binnall, an attorney for Kesari, declined to comment.

Two days after the check was written, Sorenson made a surprise appearance at a Paul campaign rally in Des Moines and announced that he was switching his endorsement. A furious Bachmann accused Sorenson of switching to Paul for money.

Sorenson denied any payments. "I was never offered a nickel from the Ron Paul campaign," he told Fox News's Megyn Kelly.

The state senator resigned from his post last year after a four-month investigation by a state court-appointed independent counsel, Mark Weinhardt, found probable cause that he violated the Iowa Senate Code of Ethics.

Weinhardt’s investigation discovered that Bachmann’s presidential campaign and leadership PAC routed nearly $60,000 to Sorenson through a consulting firm owned by Guy Short, a political consultant working for Bachmann’s PAC. The state investigation also determined that a consulting firm owned by Sorenson had been paid $73,000 by a film production company called ICT.

Federal court documents filed Wednesday confirmed that the $73,000 originated with Paul’s campaign.

“We were deeply suspicious at the time of our report that the money paid to Sen. Sorenson from the film production company was compensation for his support for Ron Paul,” Weinhardt said. “Yesterday’s events confirmed that suspicion.”