Stephanie Logan, a spokeswoman for Inspector General Michael Horowitz, declined to confirm or provide details about the scope of the inquiry, which was first reported by NBC News.
“Our general practice is to not confirm or deny the existence of any ongoing investigation,” Logan said.
Barr’s decision to overrule the line prosecutors who persuaded a jury to convict Stone on seven felony counts drew loud choruses of condemnation from Democrats and many members of the legal community, who accused the attorney general of doing a political favor for a longtime ally of the president.
Barr insisted that he stepped in only to make sure that the prosecution’s recommendation in the case was a reasonable one, given the facts. He said that the seven-to-nine-year proposal was obviously excessive under the circumstances and in view of the judge’s ultimate decision to sentence Stone to three years and four months — about half what prosecutors originally recommended.
The attorney general acknowledged that the recommendation was changed after Trump tweeted his condemnation of the original proposal, but Barr said he had told colleagues to revise the court submission before Trump weighed in.
Stone had served a few days in home confinement and was set to head to federal prison in July when Trump commuted the remainder of his sentence.
The case against Stone alleged that he sought to thwart congressional and Justice Department investigations into alleged contacts between Trump’s political supporters and WikiLeaks related to the release of email messages during the 2016 campaign. Those emails were viewed as damaging to Trump’s Democratic opponent, Hillary Clinton.
At the time of the commutation in July, Trump and Stone both vowed that his informal political counselor would continue with his appeals. The president said Stone stood a strong chance of being exonerated, but the conservative author and activist dropped his appeal last month, saying the odds of success were too remote.
Barr’s rare intervention led all four prosecutors assigned to Stone’s case to withdraw in protest, with one quitting the government altogether.
One of the prosecutors who remained in the government but dropped out of the case, Aaron Zelinsky, testified to the House Judiciary Committee in June that prior to the initial sentencing recommendation, the prosecutors came under intense political pressure to soften their recommendation for Stone. Zelinsky said the initial sentencing recommendation carefully adhered to Justice Department policy applied in thousands of cases each year.
Barr’s action in the case prompted top House Democrats and at least one influential Republican senator, Senator Chuck Grassley of Iowa, to ask Horowitz to investigate the episode.
In the past, Horowitz has written to members of Congress to confirm that he has launched inquiries in high-profile cases in which lawmakers demanded a review. It was not immediately clear why Horowitz was being more tight-lipped about the investigation into the Stone sentencing decision.
Kyle Cheney contributed to this report.