The Justice Department said Wednesday that it had opened an investigation into a 2007 plea deal negotiated by a prosecutor who is now a member of President Trump’s cabinet that kept the financier Jeffrey E. Epstein from facing federal charges related to accusations that he molested dozens of underage girls.
The inquiry came in response to requests from Senator Ben Sasse of Nebraska, who on Wednesday posted a letter on his website from Assistant Attorney General Stephen E. Boyd informing him of the investigation.
The plea deal, which was negotiated by the labor secretary, R. Alexander Acosta, when he was the top federal prosecutor in Miami, was the subject of a Miami Herald investigation that was cited by both Senator Sasse and Mr. Boyd. The letter does not mention Mr. Acosta by name.
In the letter, Mr. Boyd said the Justice Department Office of Professional Responsibilities would look “into allegations that Department attorneys may have committed professional misconduct in the manner in which the Epstein criminal matter was resolved.”
Mr. Boyd said the office “will thoroughly investigate the allegations of misconduct that have been raised and, consistent with its practice, will share its results” with Senator Sasse “at the conclusion of its investigation as appropriate.”
A lawyer for Mr. Epstein, Martin Weinberg, said Wednesday that the 2007 plea deal had been reviewed and approved by “multiple levels of the Department of Justice.”
“The agreement entered into between Jeffrey Epstein and the United States Attorney’s Office was fairly negotiated by experienced teams of attorneys representing each party,” Mr. Weinberg said in an email. “It was anything but a sweetheart deal.”
A spokeswoman for Mr. Acosta at the Department of Labor did not respond to a request for comment on Wednesday. But on Capitol Hill, Mr. Sasse, a Republican member of the Senate Judiciary Committee, celebrated the decision to open an investigation.
“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,” Mr. Sasse said in a statement on Wednesday. “The victims of Epstein’s child sex trafficking ring deserve this investigation — and so do the American people and the members of law enforcement who work to put these kinds of monsters behind bars.”
Mr. Epstein, a well-connected investor who once had a list of billionaire clients and influential friends including President Trump and former President Bill Clinton, was suspected by the F.B.I. of international sex trafficking of underage girls.
Police findings showed Mr. Epstein had engaged in sex acts with underage girls, some of them runaways or foster children, in his Palm Beach, Fla., mansion. He currently lives in the United States Virgin Islands, Mr. Weinberg said.
The Miami Herald investigation, published last year, detailed how Mr. Acosta, when he was United States attorney for the Southern District of Florida, secretly negotiated a plea deal with Mr. Epstein that granted federal immunity to him and “any potential co-conspirators.”
Under the terms of the nonprosecution agreement, Mr. Epstein pleaded guilty to state charges of soliciting prostitution from a minor. He was supposed to spend 18 months in the Palm Beach County jail.
But once in custody, Mr. Epstein was granted work-release privileges that let him leave jail for up to 12 hours a day six days a week, The Herald found. He served 13 months in jail.
Mr. Weinberg, the lawyer for Mr. Epstein, said in an email that the plea deal had been in the best interest of prosecutors.
“As a result, the government avoided litigation risks including the risks of an acquittal,” he wrote. “And Mr. Epstein went to jail, served a period of probation, lived up to each and every obligation under the agreement, and has fully conformed his conduct to the law for well over 10 years.”
The deal enraged Mr. Epstein’s accusers, who did not learn about its existence until after it was approved by a judge.
Two of his accusers, who are now adults, have filed a lawsuit against the federal government. It alleges that the deal violated the Crime Victims’ Rights Act and seeks to have it voided. On Wednesday, their lawyer, Bradley Edwards, declined to comment on the announcement about the federal investigation.