Supporters of the Islamic State rushed to celebrate Saturday night’s powerful explosion in New York’s Chelsea neighborhood that injured at least 29 people, even as no group or individual has been arrested or claimed responsibility. “The lions of the Caliphate roar in New York, we cause you pain inside your house, the carrier of the Cross,” wrote one Twitter user who went by the name “I am ISIS, come and block me.” The account was soon suspended. Another, called “The Lone Wolves,” tweeted with the Arabic hashtag #ExplosionManhattanNewYork “Oh God burn America, take revenge in the name of your oppressed...