Riots erupted for a second day on Wednesday in the northern Nigerian city of Kano, where Muslim youths defied a huge security operation and launched attacks on the Christian minority to avenge a massacre of Muslims. Doctors and mortuary personnel at Kano General Hospital confirmed that the death toll had risen to 15, as gangs targeted Christians in reprisal for a May 2 massacre by a Christian ethnic militia on the mainly Muslim market town of Yelwa, in central Nigeria, which left more than 300 dead.