WSJ
I am making no comment as to causes probable or otherwise. But in relation to the WSJ I can state that one is dealing with a completely different kind of newspaper from any of the papers in either Spain or the UK. The WSJ even now it is owned by the Dirty Digger is a very serious operation.
If the Journal writes a story with this degree of certainty it is true. They will have talked to someone connected with the investigation, the fact checkers will have confirmed that the individual concerned was in a position to know and was not some wannabe. The conversation will either have been taped or contemporaneously noted.
At a rough thought I would guess the source is in the US part of the team but purely hypothesis on my part