The Russian claimed to have access to a staggering collection of secrets that included everything from the computer code for the cyberweapons stolen from the N.S.A. and C.I.A. to what he said was a video of Mr. Trump consorting with prostitutes in a Moscow hotel room in 2013, according to American and European officials and the Russian, who agreed to be interviewed in Germany on the condition of anonymity. There remains no evidence that such a video exists.

The Russian was known to American and European officials for his ties to Russian intelligence and cyber criminals — two groups suspected in the theft of the N.S.A. and C.I.A. hacking tools.

But his apparent eagerness to sell the Trump “kompromat” — a Russian term for information used to gain leverage over someone — to American spies raised suspicions among officials that he was part of an operation to feed the information into United States intelligence agencies and pit them against Mr. Trump. Early in the negotiations, for instance, he dropped his asking price from about $10 million to just over $1 million. Then, a few months later, he showed the American businessman a 15-second clip of a video showing a man in a room talking to two women.

No audio could be heard on the video, and there was no way to verify if the man was Mr. Trump, as the Russian claimed. But the choice of venue for showing the clip heightened American suspicions of a Russian operation: The viewing took place at the Russian embassy in Berlin, the businessman said.