Thousands of contributions to Democratic presidential candidate Bernie Sanders’ campaign in January violated federal campaign finance laws, election regulators said on Thursday.

The Federal Election Commission sent a letter to the Democratic presidential candidate’s campaign committee on Thursday with a 90-page spreadsheet listing 3,457 "excessive, prohibited, and impermissible contributions."

The campaign’s January financial disclosure filing listed contributions from foreign nationals and unregistered political committees, the FEC said. Other contributions came from donors who exceeded the $2,700 per-election limit.

"Although the Commission may take further legal action concerning the acceptance of [excessive or prohibited] contributions, your prompt action to refund the prohibited amount will be taken into consideration," the FEC told the campaign.

Sanders’ campaign has relied on small-dollar individual contributions to a far greater extent than any other presidential campaign, including the Super PAC- and dark money-fueled efforts of Democratic rival Hillary Clinton.

The Vermont Senator and self-described socialist is running on a platform of transparency and campaign finance reform, contrasting his grassroots support with Clinton’s high-dollar donors and use of loopholes in federal election laws that allow her campaign to coordinate with outside groups that can accept unlimited contributions.

However, Sanders’ donors have also run afoul of federal campaign finance laws, and his financial disclosure reports have been riddled with errors.

The FEC sent a letter to the Sanders campaign earlier this month flagging an additional 1,316 "excessive, prohibited, and impermissible contributions" in the fourth quarter of 2015.

The commission also noted disbursements from the campaign that failed to include required documentation.

The Sanders campaign did not immediately respond to a request for comment.

Some of the campaign’s legal problems stem from enthusiasm for Sanders’ candidacy from foreign nationals, many of whom have publicly revealed donations to the campaign in violation of U.S. election laws.

"I am German, live in Germany and just donated to Bernie Sanders' campaign on www.BernieSanders.com simply using my credit card—Is this illegal in any way?" asked a user on the website Quora.

"UPDATE: Donation rescinded based on your answers," the user later added.