This New Yorker essay Is the F.B.I. Truly Biased Against Trump? about the FBI’s investigations of Rudolph Giuliani and Hunter Biden has a couple of telling paragraphs:
According to [FBI agent] Buma’s statement, shortly after Russia invaded Ukraine, on February 24, 2022, he was told to terminate relations with one of his most valuable sources in that field, Dynamo. The order came from both his supervisors and the F.B.I.’s Foreign Influence Task Force, and, per Buma, superiors told him that the shutdown of Dynamo was based on “highly classified information from the National Security Agency” which he could not access. They also said that it was part of a broader effort, around the time of the invasion, to close off many “sources related to Russia/Ukraine matters. ”
Judging from the tone of Buma’s statement, he had become convinced that the demand to sever ties with Dynamo was another part of a shadowy conspiracy to protect Giuliani, even at the cost of depriving American officials of potential information about the unfolding war. “I believe that the intelligence that was suppressed could easily have influenced U.S. decisions surrounding the conflict.”
It seems more likely to me that the investigation was shut down not to protect Giuliani but, rather, to hide information about what was going on in Ukraine. I can only imagine the unsavory things the CIA and related groups did in the shadows to provoke Russia’s invasion.