The Justice Department is launching an investigation into a decade-old plea deal in which critics say Labor Secretary Alexander Acosta as a federal prosecutor allowed hedge fund manager Jeffrey Epstein, who was facing sex abuse and trafficking allegations, to avoid harsh punishment.

The DOJ Office of Professional Responsibility “has now opened an investigation into allegations that department attorneys may have committed professional misconduct in the manner in which the Epstein criminal matter was resolved,” Assistant Attorney General Stephen Boyd said in a Feb. 6 letter to Sen. Ben Sasse (R-Neb.).

Boyd was responding to Sasse and other lawmakers’ calls to look into an agreement that Acosta—then a federal prosecutor in South Florida—reached with lawyers for the Miami hedge fund manager, which allowed Epstein to avoid federal sex-trafficking charges. Epstein was sentenced to 13 months in prison on lesser charges, in what Sasse recently called a “sweetheart” deal.

A November Miami Herald report on the case renewed public attention to the allegations against Epstein, who was accused of running a teen sex ring out of his Florida home. The article also brought new criticism to Acosta, who as U.S. attorney for the Southern District of Florida is said to have helped orchestrate the agreement.

Epstein’s all-star legal defense team included Harvard University professor Alan Dershowitz, former special prosecutor Kenneth Starr, and famed criminal defense attorney Roy Black.

“The Secretary welcomes the OPR’s additional review of this matter,” a Labor Department spokeswoman told Bloomberg Law Feb. 6.

“For more than a decade, this prosecution has been reviewed in great detail by newspaper articles, television reports, books, and Congressional testimony,” the spokeswoman said in an email. “Department of Justice leadership, likewise, reviewed the matter at the time, and the Department has continued to defend the Southern District of Florida’s actions across three administrations and several attorneys general on the grounds that the actions taken were in accordance with Department practices, procedures, and the law.”

Boyd’s letter comes days after DOJ Inspector General Michael Horowitz told a group of House Democrats that he doesn’t have the authority to review the Epstein case. Horowitz urged the Senate to pass a bill already passed by the House, which would shift the power to probe prosecutorial misconduct allegations to the IG from the Office of Professional Responsibility.

The pressure is ramping up from advocacy groups as well. Allied Progress, a consumer watchdog group, launched a digital ad campaign Feb. 6 calling on the Senate to approve the legislation. The organization Feb. 6 posted a video, running 40 seconds long, that uses cuts of Acosta’s labor secretary confirmation hearing, in which he discussed his role in Epstein’s case.

“It’s time to hold Secretary Acosta accountable,” the group says in the video, calling Acosta a “serial sex abuser’s dream prosecutor.”