LONDON, Oct. 14 -- A day after the prime minister said Iran's Revolutionary Guards almost certainly directed a suspected plot against RAF Fairford, the head of MI5 declined to say anything about the case, and Tehran answered by demanding the evidence.

Sir Ken McCallum, MI5's director general, used his annual threat update at Thames House on Tuesday to say that Iran has shifted toward "harder targets, like military sites." On Fairford he cited ongoing investigations, called the case "not unique" and gave no details. According to CBS News, he did not himself say Iran directed it.

That leaves two official voices saying different things. Andy Burnham gave a verdict on Monday; the spy chief gave a trend on Tuesday. What is public about the u...