The Federal Election Commission sent a warning letter to Bernie Sanders’s campaign on Thursday, accusing the Vermont senator of accepting thousands of illegal campaign contributions and threatening legal action unless the money is returned.

The letter, first obtained by the Washington Free Beacon, contains a spreadsheet detailing nearly 3,500 “excessive, prohibited, and impermissible contributions” collected by Sanders in January. Hundreds of donors listed foreign addresses, while thousands of others ran over the $2,700 limit for individual contributions.


The letter warned that the FEC “may take further legal action,” but added that “your prompt action to refund, redesignate, and/or reattribute the excessive amount will be taken into consideration.”

This isn’t the Sanders campaign’s first run-in with the FEC — a letter sent earlier this month documented more than 1,300 illegal contributions he received in the last quarter of 2015.

The allegations could be a problem for Sanders, who frequently boasts of his campaign’s success in attracting over 3.5 million small-dollar donations, contrasting his fundraising with Hillary Clinton’s reliance on rich donors and super PACs.

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