The investigation follows a Federal Election Commission (FEC) complaint filed by one of her top campaign lieutenants earlier this year that alleged improper payments to a state official in Iowa who now is the subject of a criminal probe in connection with the campaign’s use of an e-mail list of Iowa home-school families.

An attorney for Bachmann’s campaign on Monday denied any wrongdoing on her part. But the inquiry by the independent Office of Congressional Ethics (OCE) adds to a mounting list of legal problems stemming from the waning days of Bachmann’s failed presidential campaign, including an ongoing police investigation and a lawsuit over the e-mail list.

A spokesman for Bachmann’s congressional office suggested that the disclosure of the new ethics review, though it originates with staffers in her own campaign, was engineered by political adversaries.

“Unfortunately, the disclosure of the existence of this review is a predictable and politically motivated attack by Congresswoman Bachmann’s political adversaries in an attempt to disparage her reputation as a top target of the DCCC [Democratic Congressional Campaign Committee] and Democratic Super PACs,” said Bachmann spokesman Dan Kotman. “They are willing to do or say anything in an attempt to defeat her in 2014.”

William McGinley, a top GOP Washington attorney representing Bachmann’s campaign in the new inquiry and before the FEC, said, “There are no allegations that the congresswoman engaged in any wrongdoing.” He added that “We ... are confident that at the end of their review the OCE board will conclude that Congresswoman Bachmann did not do anything inappropriate.”

The new ethics probe, first reported in the Daily Beast, could lead to sanctions by the U.S. House, though few of the office’s inquiries go that far. Established in 2008, the ethics office looks into charges against House members and their staffs and makes recommendations to the House Ethics Committee.

At least three former staffers contacted by the Star Tribune say they have been contacted by the office in the past six weeks, suggesting that the probe has gone beyond the initial 30-day period for a preliminary inquiry.

A senior attorney for the ethics office said he could not confirm or deny the Bachmann investigation. Among the board’s eight members — half of which are appointed by Democrats and half by Republicans — is former Republican Rep. Bill Frenzel of Minnesota.

Frenzel declined comment when contacted by the Star Tribune last week.

The OCE, chaired by former CIA Director Porter Goss, was formed in the wake of the Jack Abramoff lobbying scandal to help depoliticize Congress’ investigations of its own members, which often have been criticized for lacking rigor. The OCE, however, has sometimes been criticized by lawmakers for pursuing complaints they considered questionable.

Members of the Congressional Black Caucus once accused OCE of “targeting” African-Americans after the office investigated then-Rep. Charles Rangel, D-N.Y., over his expenses from a Caribbean trip.

Although the office received thousands of citizen contacts in the last Congress, formal inquiries can be launched only with the agreement of two board members from different parties. In the past two years, it began 32 preliminary inquiries. Only 13 of those were transmitted for further review by the House Ethics Committee, according to the board’s most recent quarterly report.

“We don’t chase a lot of wild gooses,” said Bill Cable, a senior OCE attorney. “We find out fairly quickly if there’s something there or not.”

The allegations against Bachmann stem largely from the FEC complaint filed by Peter Waldron, a well-known evangelist who served as the campaign’s national field coordinator for outreach to Christian conservatives.

Waldron, a controversial figure who once was arrested for possession of assault rifles in Uganda, accused the Bachmann campaign in January of withholding payments to former staffers who refused to sign confidentiality agreements. He also filed papers alleging campaign finance violations involving the campaign and MichelePAC, Bachmann’s independent political action committee.

According to Waldron’s FEC complaint, the campaign improperly used MichelePAC money to pay longtime fundraising consultant Guy Short in the final months leading up to the January 2012 Iowa caucuses. Short did not return a request for comment Monday.

Waldron also has accused the campaign of concealing payments to Iowa state campaign chairman Kent Sorenson, a state senator who abruptly left the Bachmann camp to join then-U.S. Rep. Ron Paul’s insurgent campaign.

Sorenson’s attorney, Ted Sporer, noted that the Iowa Senate has set aside a separate ethics complaint brought by Waldron. “Clearly, we don’t think we did anything wrong,” Sporer said.

Sporer confirmed that the OCE has contacted him in the past month to get a statement from Sorenson. “We’re going to be cooperative and truthful,” Sporer said.

Sporer also denied allegations made in a lawsuit by Barbara Heki, a former Bachmann campaign worker who says Sorenson took an e-mail list of Iowa home-schoolers from her personal computer in the campaign’s Urbandale, Iowa, office.

Kevin Diaz • 202-383-6120