The Justice Department’s in-house watchdog found no evidence of political interference in the reduction of a prison sentence proposed for the longtime Trump ally Roger Stone in 2020, attributing the stunning reversal to “ineffectual” leadership, according to a report released on Wednesday.

The report concluded a four-year investigation into the decision by Attorney General William P. Barr in February 2020 to reduce Mr. Stone’s proposed sentence to about three years, after initially recommending seven to nine. The episode incited accusations of political interference and prompted four prosecutors in the case to resign.

The decision to seek a lighter sentence was announced after President Donald J. Trump sharply criticized the harsher sentence initially proposed by career prosecutors after Mr. Stone was convicted in 2019 for lying to Congress during its investigation into Mr. Trump’s connections to Russia. He denounced the sentencing recommendation as a “miscarriage of justice.”

But the inspector general, Michael E. Horowitz, concluded that Mr. Barr had argued that the more stringent sentence was unreasonably harsh before Mr. Trump expressed those sentiments. Mr. Barr was blindsided by a subordinate who had led him to believe the department would request a lesser prison term, Mr. Horowitz wrote.

“Barr had articulated his position about the sentencing recommendation both before and shortly after the first sentencing memorandum was filed, and before the president’s tweets,” Mr. Horowitz wrote in the 85-page report.

Mr. Stone never served any time; Mr. Trump commuted his sentence before leaving office.

Mr. Barr did not immediately respond to a request for comment. But he has repeatedly denied that his actions were motivated by partisanship or political pressure.