At least 20 people were killed in Nigeria on Monday when a suicide bomber blew himself up at a Shiite religious ceremony in Potiskum, a town in the northeastern part of the country. No group has claimed responsibility for the bombing, but given its history of violence against Shiites, Boko Haram, the Sunni militant group, is thought to have organized the attack. Boko Haram claims that Shiites, a minority in Nigeria, are non-Muslims.

According to a report by the BBC, witnesses said the bomb exploded when a crowd was marching to mark Shura, a day when Shiites mourn the death of Hussein, a grandson of the Prophet Muhammad. The Shiite worshippers were outside a seminary in the Tsohuwar Kasuwa area of the city after visiting the palace of the local emir, Al Jazeera reported.

The town of Potiskum, in Yobe state, was placed under emergency rule last year when the military launched a operation to fight Boko Haram. More than 1,000 people have been killed in the region since the emergency rule was put in place.

The attack on Monday comes after Boko Haram denied the existence of  a ceasefire which the government claimed last week had been established. In a video released Saturday, the group's leader denied there was such a deal and said the negotatior on the deal who spoke with the government officials about agreeing to end the violence did not represent the militant group. "We will not spare him and will slaughter him if we get him," Abubakar Shekau, the leader, said of the negotiator.

He also claimed the school girls who were abducted in April were converted to Islam and married off. "Don't you know the over 200 Chibok schoolgirls have converted to Islam?" he said. "They have now memorized two chapters of the Quran."