– U.S. Sen. Al Franken has relinquished $40,822 in donations from a Boston law firm under investigation by federal regulators for linking employee bonuses to campaign contributions.

The Minnesota senator’s office confirmed this week that the money, which Franken deposited to the U.S. Treasury, came from Thornton Law Firm. The admission came after the Federal Election Commission (FEC) sent Franken a letter asking him to identify the donors.

Democratic organizations and politicians have returned more than $1 million from Thornton since an investigation by the Boston Globe and Center for Responsive Politics revealed that the firm reimbursed partners for millions in political contributions to Democratic politicians.

The Democratic Senatorial Campaign Committee, Democratic presidential nominee Hillary Clinton, Sen. Elizabeth Warren of Massachusetts and former Sen. Russ Feingold of Wisconsin all gave back donations from Thornton Law Firm employees in recent months.

The firm’s lawyers and their spouses — among the largest political donors in the nation — have been among Franken’s top contributors. They donated to Franken’s 2008 campaign, when he defeated Republican Norm Coleman by one of the closest margins in Senate history.

They also gave to him during the 2014 election cycle, according to the Center for Responsive Politics.

Franken returned $40,822 to Thornton Law Firm in Boston.

Franken’s spokesman said the senator turned over the contributions after learning from news reports that the law firm may have improperly reimbursed employees for their donations.

He has until April 3 to report the source of the donations to the FEC.

The FEC and the U.S. Attorney’s Office in Boston are investigating the law firm’s contribution practices. Thornton Law Firm, which specializes in asbestos litigation, has maintained that it acted legally.

Election law expert Brett Kappel said that politicians aren’t required to give back such campaign contributions unless a ruling determines that they are illegal, but that officials often return them when there’s a credible allegation of wrongdoing.

“They don’t want to be associated with anyone who’s violating the law,” said Kappel, of the Washington, D.C. law firm Akerman LLP.

He noted that Franken’s relinquished contributions were significant for a Senate campaign, and that the combined amount from politicians around the country could amount to the largest such reimbursement in history.