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 Mr. 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.”