A Shia Muslim procession in the Nigerian state of Kano was hit by a suicide bombing attack on Friday, killing at least 21 people and injuring many others.

There has been a wave of bombings of this nature in Nigeria and northwestern Africa in the past several months, and the group's leaders expressed that it expected to be targeted. In the last week alone, two other bombings have occurred, one in Maiduguri and the other in northern Cameroon, according to the Guardian.

The Islamic Movement of Nigeria (IMN) has blamed Boko Haram, a Sunni militant group, for Friday's attack, according to the BBC. Boko Haram has terrorized the area for the past six years, killing upwards of 15,000 people. The group believe Shia Muslims to be heretics that must be killed. Boko Haram has been linked to the Islamic State, reports International Business Times.

During the march, a man ran into the crowd of thousands of people and quickly detonated an explosive before he could be apprehended. According to an anonymous witness, the bomber's accomplice was arrested immediately and confessed that he was sent by Boko Haram. Even after the chaos induced by the blast, the procession continued forward along the bloodstained road, reports The Guardian.

The procession runs annually for seven days. It runs from Kano to Zaria, where the IMN is headquartered. 

"We are not surprised that we've been attacked because this is the situation all over the country. This will not deter us from our religious observance," Mohammad Turi of the IMN and a leader of the march, according to the BBC.