Jean-Marie Guenois, the religion correspondent of Le Figaro, may be quoted as saying that "Bishop Williamson is probably the source of the leak. Of note in the article are le Figaro's veerification of the authenticity of the letters, and the assertion that Bishop Williamson "probably facilitated" the leaks.
"This leak, probably facilitated by the famous Bishop Williamson - author of negationist statements denying the past [the Holocaust]- and now opposed to this agreement, arrived in a very specific context.... the announcement of this landmark agreement could come around the Feast of Pentecost. Unless, of course, there is a last-minute reversal. It would not in any case be from the Holy See, as Benedict XVI wishes to conclude [the agreement]. Neither from Bishop Fellay, which is demonstrated in commitment letter [to the other three bishops]. In other words, despite the internal division in which he faced for a long time, things are about to arrive".
The full article at Le Figaro may be read here.