A special counsel appointed in President Donald Trump’s administration issued a highly critical report on how the FBI handled allegations linking Mr. Trump to Russia in 2016, ending four years of work Monday after having lost the two criminal cases he took to trial.

In his 306-page report, John Durham, the former top federal prosecutor in Connecticut, repeated prior criticisms faulting the Federal Bureau of Investigation on a number of points.

He said the bureau swiftly pursued a vague tip about potential contacts between a Trump campaign aide and Russia authorities in July 2016, even though, the report says, the bureau had no other information in its files to corroborate any such contact. The Justice Department’s inspector general in 2019 found similar flaws in the FBI inquiry but found the investigation was justified.

He concluded the FBI was more cautious and skeptical of allegations of foreign influence on the Clinton campaign than on the Trump campaign in 2016. According to the report, the bureau didn’t aggressively pursue evidence of two instances in which foreign governments were potentially planning to contribute to Hillary Clinton’s presidential campaign to gain influence. The speed with which the FBI opened the investigation into the Trump campaign “based on raw, unanalyzed, and uncorroborated intelligence also reflected a noticeable departure from how it approached” those other allegations, it said. The FBI provided briefings to the Clinton campaign, the report said, an approach it said stood in contrast to the lack of such briefings provided to the Trump campaign.