Boko Haram claimed responsibility on Saturday for the wave of strikes. The sect has killed hundreds in the north of Africa's most populous nation in the last year.
A spokesman for Boko Haram contacted reporters in the northeastern city of Maiduguri, where the sect is based, to claim responsibility for Friday's bombings. Copies of a letter apparently from the group were also dropped around Kano.
The letter, written in the Hausa language spoken in northern Nigeria, said the attacks were retribution for police arrests and killings of members of the sect.
Boko Haram became active around 2003 in the northeast state of Borno but its attacks have spread into other northern states, including Yobe, Kano, Bauchi and Gombe.
Boko Haram, a Hausa term meaning "Western education is sinful," is loosely modeled on Afghanistan's Taliban.
