Washington, D.C. – U.S. Sen. Al Franken has turned over $40,822 in donations from a law firm in Boston as federal regulators are investigating the firm for linking employee bonuses to campaign contributions.

Democratic politicians and organizations have returned more than $1 million from Thornton Law Firm since an investigation by the Boston Globe and Center for Responsive Politics. That investigation revealed that the firm had reimbursed its partners for millions of dollars in political contributions, exclusively to Democratic causes.

Hillary Clinton, Sen. Elizabeth Warren of Massachusetts, former Sen. Russ Feingold of Wisconsin, and the Democratic Senatorial Campaign Committee have all returned donations to employees of Thornton in recent months.

The Boston Globe reports that Thornton was the 11th-ranked law firm nationally for political contributions in 2014, even though it is not among the top 100 largest even in Massachusetts. Bonuses were distributed to employees as soon as the day after donating.

The Star Tribune reports that the firm’s lawyers and their spouses are among Franken’s top contributors. They donated to Franken’s 2008 campaign when he defeated Republican Norm Coleman by a mere 312 votes.

Thornton Law Firm also donated to Franken’s campaign in the 2014 election cycle.

Franken’s spokesman told the Star Tribune said that the senator returned the contributions after learning of the improper reimbursements. Franken has until April 3 to report the sources of the donations to the Federal Elections Commission.

The combined amount of returned contributions around the world could end up being the largest type of reimbursement in history.