Labour secretary nominee Alexander Acosta is expected to face questions at his Senate confirmation hearing about an unusual plea deal he oversaw for a billionaire sex offender while U.S. attorney in Miami.

Acosta has won confirmation for federal posts three times previously, but he has never faced scrutiny on Capitol Hill for his time as U.S. attorney.

Critics, including attorneys for some underage victims of financier Jeffrey Epstein, say the plea agreement was a "sweetheart deal" made possible only by Epstein's wealth, connections and high-powered lawyers. Acosta has defended his decisions as the best outcome given evidence available at the time.

"Some may feel that the prosecution should have been tougher. Evidence that has come to light since 2007 may encourage that view," Acosta wrote in a March 2011 letter to media outlets after leaving the U.S. attorney's office. "Had these additional statements and evidence been known, the outcome may have been different. But they were not known to us at the time."

Senate aides from both parties expect Democrats to raise the case during Acosta's confirmation hearing Wednesday as an example of him not speaking up for less-powerful people. The aides spoke on condition of anonymity because they weren't authorised to speak publicly.

Sen. Patty Murray, the leading Democrat on the committee, said in a statement she met with Acosta on Thursday and is concerned about whether he would "stand up to political pressure" and advocate for workers as labour secretary. Unlike Trump's original choice for labour secretary, Andrew Puzder, Acosta is expected to win confirmation.

The Florida International University law school dean was nominated after Puzder, a fast-food executive, withdrew over his hiring of an undocumented immigrant housekeeper and other issues.

Acosta, 48, has previously won Senate confirmation as Miami U.S. attorney, head of the Justice Department's civil rights division and the National Labor Relations Board.

He declined comment when asked about the Epstein case this week.

Epstein, now 64, pleaded guilty in 2008 to Florida charges of soliciting prostitution and was sentenced to 18 months in prison, of which he served 13 months. Epstein was also required to register as a sex offender and pay millions of dollars in restitution to as many as 40 victims who were between the ages of 13 and 17 when the crimes occurred.

According to court documents, Epstein paid underage girls for sex, sexual massages and similar acts at a Palm Beach mansion he then owned as well as properties in New York, the U.S. Virgin Islands and New Mexico. Prosecutors say he had a team of employees to identify girls as potential targets.

After an investigation by local police, Palm Beach prosecutors decided to charge Epstein with aggravated assault, which would have meant no jail time, no requirement that he register as a sex offender and no guaranteed restitution for victims.

Unhappy local investigators went to Acosta's office, which opened a federal probe and eventually drafted a proposed 53-page indictment that could have resulted in a sentence of 10 years to life in prison for Epstein, if convicted. With that as leverage, a deal was worked out for Epstein to plead guilty to state prostitution solicitation charges and the federal indictment was shelved.

It didn't stop there. Epstein's lawyers worked out an unusual and secret "non-prosecution agreement" to guarantee neither Epstein nor his employees would ever face federal charges.

Well-known Miami defense lawyer Joel DeFabio, who has represented numerous defendants in sex cases, said he had never heard of such an agreement before Epstein's came to light. DeFabio said he has had clients with far less egregious sex charges — and far less wealth — who were sentenced to 10 or 15 years behind bars. DeFabio tried to use the Epstein case to argue for more lenient sentences.

"There still has been no clear explanation as to why Epstein received such preferential treatment," DeFabio said. "This thing just stinks. The elite take care of their own."

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The non-prosecution agreement became public in a related civil case, leading two Epstein victims — identified only as Jane Does No. 1 and 2, to file a victims' rights lawsuit claiming they were improperly left in the dark about the deal. The lawsuit, which is still pending, seeks to reopen the case to expose the details and possibly nullify the agreement.

Other victims have come forward, including one woman who claimed as a teenager that Epstein flew her around the world for sexual escapades, including encounters with Britain's Prince Andrew. Buckingham Palace has vehemently denied those claims.

The Justice Department's position in the victims' rights lawsuit is that since no federal indictment was ever filed, the victims were not entitled to notification about the non-prosecution agreement. Settlement talks last fall went nowhere.

"There will not be a settlement. That case will eventually get to trial," said Bradley Edwards, attorney for the two Jane Doe victims.

In his 2011 letter, Acosta defended his decisions as the best possible outcome.

"Our judgment in this case, based on the evidence that was known at the time, was that it was better to have a billionaire serve time in jail, register as a sex offender and pay his victims restitution than risk a trial with a reduced likelihood of success," Acosta wrote. "I supported that judgment then, and based on the state of the law as it then stood and the evidence known at the time, I would support that judgment again."