PlameGate & the Questioning of Dick Cheney

(CNN) — Vice President Dick Cheney has been interviewed by federal prosecutors investigating a leak that disclosed the identity of an undercover CIA operative, a Bush administration official told CNN.

John King of CNN also notes that the New York Times broke this news on June 5 in an article by David Johnston who notes:

It is not clear when or where Mr. Cheney was interviewed, but he was not questioned under oath and he has not been asked to appear before the grand jury, people officially informed about the case said.

So why was Mr. Cheney not under oath during this questioning?