At first the bomber was assumed to be a “lone wolf,” then a “known wolf.” Then British police arrested someone in connection with last night’s attack. Now they have intelligence that a second attack may be impending, pushing the terror threat level to “critical” for the first time in 10 years. How big is this cell?

The practical effect of raising the threat level is that the British military will be deployed to support the police in guarding “key sites” around the country. That’s not going to make libertarians happy, but luckily for Theresa May there aren’t many of those left in the UK. It’s a smart hedge against the risk of a follow-up attack whether one is truly likely or not. She’s 16 days out from a national election; if a second bomber strikes, the narrative will shift from “those ISIS bastards” to “those ISIS bastards — but why was the government caught off-guard?” With boots on the ground across Britain, she can say that she’s doing everything possible to prevent a second strike.