WASHINGTON — The Justice Department has launched an investigation into how federal government lawyers handled the case of a wealthy Florida man accused of having sex with underage girls, a DOJ official confirmed Thursday.

One of those lawyers, Alex Acosta, is now President Donald Trump's labor secretary. But he was the U.S. Attorney for the Southern District of Florida when key decisions about the case of Jeffrey Epstein were made. Prosecutors looked into allegations that Epstein abused dozens of teenage girls in his West Palm Beach mansion in the early 2000s. He eventually pleaded guilty to state charges involving a single victim in 2008.

But according to a civil lawsuit filed by other women who said they were among Epstein's victims, one of the conditions of that deal was that a much larger federal investigation into Epstein and the people who helped him in his scheme would be dropped. Epstein, 66, served 13 months in jail and was allowed to leave almost every day through a work release program.

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An investigation by The Miami Herald said that Acosta helped engineer the deal involving only state charges and agreed that it would be kept secret from other victims until it was presented in court, denying them a chance to object. The newspaper's reporting led members of Congress to seek a Justice Department investigation. Sen. Ben Sasse, R-Neb., called the Epstein plea deal an "epic miscarriage of justice."

On Thursday, the Justice Department notified Sasse that the department's 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." The letter does not mention Acosta by name.

In response, Sasse said, "The victims of Epstein's child sex trafficking ring deserve this investigation" and commended the Justice Department and William Barr, Trump's nominee for attorney general, who said during his confirmation hearing that he would look into the allegations of misconduct.

"Jeffrey Epstein is a child rapist and there's not a single mom or dad in America who shouldn't be horrified by the fact that he received a pathetically soft sentence," Sasse said.

A Labor Department spokesperson said Acosta welcomes the Office of Professional Responsibility review. She said the Justice 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."

Acosta has said previously that the case against Epstein became stronger after the state conviction because more victims began speaking out. "The bottom line is this: Mr. Jeffrey Epstein, a billionaire, served time in jail and is now a registered sex offender. He has been required to pay his victims restitution, though restitution clearly cannot compensate for the crime."

Mimi Rocah, a former federal prosecutor and NBC News analyst, said it isn't clear how the Justice Department could discipline a former government lawyer.

"OPR's fact-finding abilities are somewhat limited because it doesn’t have subpoena power or investigators. More may need to be done to ensure that Congress and the public find out the facts," Rocah said.