Lagos Seven people have been arrested in connection with the death of a Muslim cleric who had criticised Boko Haram, the country's secret police said on Monday.
Adam Albani was was shot dead with his wife and son on February 1 in the northern city of Zaria, as he drove home from teaching a theology class, immediately raising suspicions that the militants were behind the killing.
The 54-year-old had previously criticised the group and given his support to the Nigerian military campaign against them.
The Department of State Services (DSS) said seven people, including the alleged mastermind of the attack and one of the gunmen, were detained between February 13 and February 21.
"Investigation and confessions by suspects revealed that Albani's pro-Western posture and his preaching are contrary to the Boko Haram ideology," DSS spokeswoman Marilyn Ogar said in an emailed statement.
A previous attempt to kill the cleric on January 23 failed but he was tracked just over a week later to a theological centre, trailed and then killed "in cold blood", Ogar said.
The DSS said that 42-year-old Yakub Abdullahi, who allegedly took part in Boko Haram's uprising in 2009, was "the arrowhead and mastermind" of the killing and procured the two AK-47 assault rifles used.
A 22-year-old suspect, Bilyaminu Usman, is said to have carried out the shooting and admitted firing his entire magazine of ammunition into the victims.
The others are suspected of either knowing about the plot or providing active support such as transporting the weapons, the statement read.
"(The) investigation is still ongoing with a view to unveiling other members of the sect... The suspects will soon be charged," Ogar added.
Boko Haram, which wants to create a strict Islamic state in northern Nigeria, have been waging a bloody insurgency since 2009.
Targets have included schools teaching a so-called "Western" curriculum, Christian churches and clergy as well as military and government targets, killing thousands.
Albani was killed a day after gunmen shot dead a Christian pastor in northeastern Adamawa state and a week after at least 26 church-goers were killed in the same area.
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