LONDON—British police arrested an 18-year-old man in the southern port of Dover on Saturday in a “significant” development in the hunt for the people behind a London commuter train bombing that injured 30 people a day earlier.
Prime Minister Theresa May put Britain on the highest security level of “critical” late on Friday, meaning an attack may be imminent, and soldiers and armed police deployed to secure strategic sites and hunt down the perpetrators.
