A warning letter was sent out by the Federal Election Commission (FEC) to presidential hopeful Vermont senator Bernie Sanders. This comes after it has been found out that he accepted thousands of illegal campaign contributions.

In a report by WND, the letter threatened Sanders that there will be legal action if he will not return the money. It also has inclusions of a spreadsheet that details around 3,500 contributions, which were said to be "excessive, prohibited, and impermissible." These were reportedly collected by Sanders and his team last January. Some of the donors' addresses listed were foreign while others had a $2,700 limit for individual contributions.

National Review also reported that the letter specifically said that FEC could take further legal action but it could be avoided if and when Sanders will have a "prompt action to refund, re-designate, and/or reattribute the excessive amount."

This is not the first time that Sanders had butt heads with FEC. Earlier this February, another letter was sent to him and his campaign team. The letter said Sanders again received 1,300 illegal contribution in the last quarter of 2015.

Sanders did say in the past that his campaign funds are brought in by 3.5 million small-dollar donations. He pointed out that these small donations are what makes his campaign a success as opposed to his opponent, Democratic frontrunner Hillary Clinton, relying on rich donors and super PACs.

The campaign team of Sanders must respond to the letter before March ends or they will face "audit or enforcement action" by FEC, Daily Caller reported. Sanders' campaign spokesman Michael Briggs said these kinds of inquiries are normal and expected as the campaign has been receiving small donations.

During the South Carolina primary, Clinton won in a landslide. The votes for her have taken more than 70 percent. 

Sanders and his team have taken it to Twitter to comment on the success of Clinton. He wrote on Sunday: "Bernie Sanders can't do this alone. We've gotta do it together through a strong grassroots movement."