What we know and don’t know about the attack in London on Saturday; a discussion of the political context; and what role the Islamic State might have played. Plus: a brief history of back channels. Guests: Rukmini Callimachi, who covers the Islamic State for The Times; Katrin Bennhold, a reporter based in London; David E. Sanger, chief Washington correspondent. For more information on today’s episode, visit nytimes.com/thedaily. And as mentioned on today’s show, we’re offering listeners a free trial of a New York Times digital subscription. Visit nytimes.com/dailytrial to sign up.