LONDON — Britain has been consumed for three years by a growing list of investigations into pedophile rings, but now there are growing doubts about the credibility of someone the police regarded as a major witness in one of the most explosive of these cases, which supposedly involved high-ranking politicians, intelligence chiefs, army generals and even a former prime minister.

Some recent press reports have questioned the credibility of the witness in the case, a man known only by his pseudonym, Nick, who says that he was abused by the Westminster V.I.P. ring for nine years and that he witnessed three murders. The Metropolitan Police in London issued a statement two weeks ago, saying that one of its senior officers had been wrong to describe Nick’s account as “credible and true” before the investigation was concluded.

Now the BBC, which has spent much of this year digging into the abuse claims and broadcast its findings on Tuesday night, has raised further questions about the case.

The program concluded that at least one of the three murders Nick said he witnessed cannot have happened as described. Two other men identified as victims of the Westminster ring by an activist supporting Nick’s claims told the BBC program, “Panorama,” that they could not corroborate the claims.