Paul Wood of the BBC reports on the sources of the story. There seem to be four of them, but its not clear that they are independent. H/T Kevin Drum:
Claims about a Russian blackmail tape were made in one of a series of reports written by a former British intelligence agent. As a member of MI6, he had been posted to the UK's embassy in Moscow and now runs a consultancy giving advice on doing business in Russia. He spoke to a number of his old contacts in the FSB, the successor to the KGB, paying some of them for information.
....The former MI6 agent is not the only source for the claim about Russian kompromat on the president-elect. Back in August, a retired spy told me he had been informed of its existence by "the head of an East European intelligence agency".
Later, I used an intermediary to pass some questions to active duty CIA officers dealing with the case file — they would not speak to me directly. I got a message back that there was "more than one tape", "audio and video", on "more than one date", in "more than one place" — in the Ritz-Carlton in Moscow and also in St Petersburg — and that the material was "of a sexual nature". The claims of Russian kompromat on Mr Trump were "credible", the CIA believed.
....Last April, the CIA director was shown intelligence that worried him. It was — allegedly — a tape recording of a conversation about money from the Kremlin going into the US presidential campaign.
It was passed to the US by an intelligence agency of one of the Baltic States. The CIA cannot act domestically against American citizens so a joint counter-intelligence taskforce was created....A lawyer — outside the Department of Justice but familiar with the case — told me that three of Mr Trump's associates were the subject of the inquiry. "But it's clear this is about Trump," he said.
Trump has shown no inclination to show his tax returns. They might shed light on the truth or falsity of the allegations.