From Tatters of the King
A senior figure in the Brothers of the Yellow Sign this twisted old man runs an import/export business in Milan. His office was given as a contact address by Malcolm Quarrie. When they visited, the investigators met Colombo and Thomas Villiers, but Colombo was initially unfriendly and then downright abusive to them.
There is some suspicion that Colombo was in some way responsible for Villiers ending up in the canal after he caused the death of Dr. Bartlett, but there is no evidence to support this.