Boko Haram, the radical Islamist sect behind a recent surge of violence in Nigeria, launched a series of attacks Friday that left at least 185 people dead in Kano, the country’s second-largest city. The attacks struck multiple security buildings as well as the regional police headquarters, and were the deadliest yet by the militant organization.

The group, which aims to overthrow the Nigerian government and impose Sharia law, has grown increasingly violent, with its August 2011 bombing of the United Nations building in Abuja, the capital, as well as its attacks on churches raising alarm among international observers. ...