How then, did he--and why was he allowed--to pass them, only to be assailed in ascending the steps?
Burchard, presumably, did not himself see these horsemen; certainly he cannot have seen them escorting the murderers to the Pertusa Gate.
Therefore he must have had the matter reported to him.

Naturally enough, had the horsemen existed, they must have been seen.

How, then, does it happen that Capello did not hear of them?
nor the Florentine ambassador, who says that the murderers were four, nor any one else apparently?
To turn for a moment to the Florentine ambassador's letters upon the subject, we find in this other Capello--Francesco Capello was his name--accounts which differ alike from Paolo Capello's and from Burchard' stories.

But he is careful to say that he is simply repeating the rumours that are abroad, and cites several different versions that are current, adding that the truth of the affair is not known to anybody.

His conclusions, however, particularly those given in cipher, point to Cesare Borgia as the perpetrator of the deed, and hint at some such motive of retaliation for an attempt upon his own life as that which is given by the ambassador of Venice.
There is much mystery in the matter, despite Gregorovius's assertion to the contrary--mystery which mere assertion will not dissipate.