Who Was the Mystery Witness Invoking the Fifth Before the House Ethics Committee?

According to a letter sent today by the House Ethics Committee to the Speaker, outside counsel Billy Martin has spent a good deal of time reviewing “allegations that this Committee violated due process rights or rules attaching to Representative [Maxine] Waters.” However, Martin has been unable to complete the due process review because one “necessary witness” has refused to cooperate with Martin’s investigation, and, when the witness was subpoenaed to testify, “communicated to the Committee that the witness would refuse to answer questions on the basis of the witness’s Fifth Amendment privilege.”

The letter states that Martin “has reviewed tens of thousands of pages of documents, and has interviewed current and former Committee Members as well as current and former Committee staff. Each current and former Committee Member and current employee, who was requested for interview, fully cooperated with Mr. Martin.” (emphasis added).

Following the rule of expressio unius est exclusio alterius (look it up), I would say that the witness invoking the Fifth is a former House Ethics Committee staffer. Any bets on who it is?

Update: According to John Bresnahan of Politico, the attorney for Morgan Kim and Stacy Sovereign says that they “both testified willingly and voluntarily before the special counsel and answered all questions asked of them.” So who does that leave?