It was an incident that seemed to bolster President Donald J. Trump’s claim that election workers were scheming against him: In September 2020, the Justice Department announced it was investigating the dumping of pro-Trump absentee ballots in Pennsylvania.

But prosecutors quickly determined it was an innocent error. A mentally impaired seasonal elections employee had mistakenly believed that nine ballots were invalid, and tossed them in a dumpster — yet the fact that no charges would be brought was not made public until well after Joseph R. Biden Jr. defeated Mr. Trump.

On Thursday, nearly four years later, the Justice Department’s in-house watchdog issued a scathing report criticizing the attorney general at the time, William P. Barr, and David Freed, then a Trump-appointed U.S. attorney in Pennsylvania, for publicly disclosing a continuing criminal investigation and allowing a mistaken perception of the incident to linger during an election.

The inspector general, Michael E. Horowitz, said he was particularly “troubled” by Mr. Barr’s decision to brief Mr. Trump on the inquiry, which, in turn, might have encouraged him to make false and exaggerated claims about election security.

“Department leadership was aware of information that substantially undercut” the narrative that election workers had thrown out ballots cast by Trump voters, he wrote in the 82-page report, which included four recommendations for reforms.

Mr. Barr and Mr. Freed chose not to tell the public before the election that “the subject of the investigation was mentally impaired, appeared to have discarded the ballots by mistake and would likely not be criminally charged,” Mr. Horowitz said.