At least 50 people were killed when a teenage suicide bomber blew up his vest packed with explosives in a crowded mosque in Nigeria's Mubi town, police said.A police official said the dawn attack took place after the prayers at the mosque in Adamawa state, Xinhua news agency reported."We have 50 dead and we are trying to get the exact number of injured," said police official Othman Abubakar.Many of the injured were in critical condition. No group has claimed responsibility for the horror but Islamist militants Boko Haram typically target crowded places in northern Nigeria. Some 20,000 people have been killed in Boko Haram's eight-year insurgency.At least 45 people were killed in a suicide attack in the same state in December last year. In that attack two female suicide bombers detonated their explosives on a busy market.