A pair of Senate Democrats is accusing Jeffrey Epstein and his lawyers of 'unconscionable' efforts to 'smother' a federal investigation – and is demanding to know why now Labor Secretary Alex Acosta gave the convicted sex offender a sweetheart deal anyway.

Sens. Patty Murray of Washington and Tim Kaine of Virginia of the Health, Education, and Labor Committee raised the issue of alleged 'witness tampering' and other alleged obstructive conduct in a letter to the Justice Department Tuesday.

They point to charges leveled by federal prosecutors in New York, who on Monday unsealed a 14-page indictment accusing the wealthy hedge funder of trafficking in underage girls as young as 14 years old.

Secretary of Labor Alex Acosta is accused of knowing about alleged witness tampering by Jeffrey Epstein. Prosecutors in New York say Epstein's lawyers put together a legal 'proffer' where he would admit to some such conduct

Acosta, a former U.S. attorney in Florida, is under fire for having negotiated the deal that let Epstein avoid federal prosecution and serve just 13 months in a Florida county jail.

'Of particular importance to your office's inquiry into the conduct of Florida DOJ attorneys, SDNY attorneys have also alleged additional unconscionable and illegal activity by Epstein and his lawyers in their successful efforts to smother the initial prosecution in the Southern District of Florida, including 'witness tampering, harassment, or other obstructive behaviors,'' the senators wrote the head of DOJ's Office of Professional Responsibility.

They then referenced recent filings in the Souther District of New York.

'In its bail memorandum, SDNY attorneys cite discussions between Epstein's lawyers and the Florida DOJ lawyers that demonstrate DOJ knew at the time about issues of obstruction, harassment, and witness tampering,' they continue. 'Then-United States Attorney Alexander Acosta subsequently did not bring charges for these offenses, once again illustrating the inequities in our justice system in favor of the rich and powerful.'

Acosta, seen here with President Trump, is under fire for his role in the Epstein case, and addresses the media Wednesday

Legal star Alan Dershowitz, who helped represent Epstein, has defended his conduct, and told CBS he would fight to get an even better deal if given the chance

Sens. Patty Murray of Washington and Tim Kaine of Virginia wrote the Justice Department seeking information about an internal probe into the Epstein deal

According to the memo filed by New York prosecutors opposing bail for Epstein, 'there were credible allegations that the defendant engaged in witness tampering, harassment, or other obstructive behaviors.'

'In fact, according to publicly-filed court documents, there were discussions between prosecutors and the defendant's then-counsel about the possibility of the defendant pleading guilty to counts relating to 'obstruction,' as well as 'harassment,' they wrote.

According to the government, Epstein's lawyer provided a 'proffer' to prosecutors – never agreed to – where Epstein would admit he ''attempted to harass both [redacted] delay and hinder their receipt of a [redacted] to attend an official proceeding.'

The proffer would also have had Epstein state that he 'flew with both [redacted] to the United States Virgin Islands rather than to an airport in New Jersey in order to attempt to delay their receipt of what Mr. Epstein expected to be a [redacted].'

Sens. Patty Murray and Tim Kaine wrote the Justice Department's Office of Professional Responsibility Tuesday demanding an update on an internal probe

Another possible obstruction plea 'could rely on the incident where Mr. Epstein's private investigators followed [redacted] father, forcing off the road.

'The defendant's apparent previous willingness to obstruct a federal investigation, harass or tamper with witnesses, and hire private investigators that 'forc[ed] off the road' the father of an individual relevant in the investigation is alarming,' prosecutors wrote.

Senators Murray and Kaine are seeking an update on an internal investigation into Acosta's role by the agency's Office of Professional Responsibility.