when the object of a conspiracy is to obstruct justice, mislead law enforcement officers, or commit similar offenses by making false statements to investigating officers, truthful statements made to such officers designed to lend credence to the false statements and hence advance the conspiracy are not rendered inadmissible by the Confrontation Clause. A contrary reading of the rule would result in obvious and unacceptable impediments to prosecuting cases like this one, in which the very object of the charged conspiracy is for the defendants to mislead investigators by responding falsely to the investigators' questions in a structured setting, fully aware that their responses might be used in future judicial proceedings.