What does the U.S. government think about these terrorism reports? Are they accurate or overblown? I can’t offer any guidance for readers, because officials at the White House and various agencies still wouldn’t comment as of late Tuesday. The new threats seem to involve targets in Europe, but U.S. officials are also focused on operations in Yemen to capture or kill militant American-born cleric Anwar al-Aulaqi.

Americans shouldn’t obsess constantly about terrorist threats. It’s bad for our national psyche. But by the same token, if terrorism becomes an unmentionable subject — with officials across the government clamming up — then we’re living in another sort of artificial world.

Evans explained to his audience in London why it is important to have a measured, open discussion: “In recent years we appear increasingly to have imported from the American media the assumption that terrorism is 100 percent preventable, and any incident that is not prevented is seen as a culpable government failure. This is a nonsensical way to consider terrorist risk and only plays into the hands of the terrorists themselves.”