The Justice Department has launched a probe into an unusual pattern of maxed-out contributions to Rep. Jim Renacci (R-Ohio) and Ohio state Treasurer Josh Mandel, the Republican Senate nominee.

The inquiry stems from a 2011 report in the Toledo Blade showing that 17 employees of Suarez Corporation Industries, an Ohio marketing firm, had donated to Renacci, Mandel or both. Sixteen of the 17 had given $5,000 — the legal maximum — including Benjamin Suarez, the company’s founder, who maxed out to both candidates.

Many of the contributions came from donors with low-level positions and low home values, and many had never given to a federal campaign before maxing out for Mandel and Renacci. The pattern led Democrats to question whether the contributions were part of a straw-donor scheme to reimburse employees who donated to the campaigns.

Both campaigns firmly maintained that the donors, not the campaigns, are the subjects of federal inquiry.

“The campaign is aware of the investigation and is fully cooperating,” said Mandel spokesman Travis Considine. “Neither the campaign nor anyone associated with it is a subject of the investigation.”

Considine added later Monday that the campaign was setting aside about $100,000 of donations from Suarez employees until the federal investigation is complete. He said the campaign may return the funds or donate them to charity.

A former Marine and first-term treasurer, Mandel is the Republican nominee challenging Sen. Sherrod BrownSherrod Campbell BrownSenate Dems press for info on any deals from Trump-Putin meeting Dems call for hearings on Trump’s CFPB nominee to be put on hold Senate Dems build huge cash edge in battlegrounds MORE (D-Ohio). Renacci faces Rep. Betty Sutton (D-Ohio) in a member-on-member match-up resulting from redistricting.

Renacci’s campaign told The Hill it was contacted about four or five months ago by a Justice Department attorney seeking information about the dates of contributions, and the campaign’s treasurer handed over the relevant records. The campaign said that was the last it had heard about the inquiry, which was first reported Monday by The New Republic.

A reporter for the magazine approached several of the firm's who had donated to Renacci or Mandel. All said they had given to the campaigns of their own free will.

“To our knowledge, no donations made to our campaign were made improperly,” said Renacci spokesman James Slepian. “If we find out they were, they will be returned.”

- This story was posted at 3:33 p.m. and has been updated.