Police have started investigating the alleged involvement of militant suspect Abdul Gaffer in a case filed for grenade attacks at two mosques inside Bangladesh Navy's Issa Khan Base in Chittagong in 2015.

Police said Abdul Mannan, his younger brother, carried out the attack after being instigated by Gaffer, also a JMB leader in Tangail.

Gaffer, now in Kashimpur jail, was involved in the killing of former PDB chairman Khizir Khan in Dhaka, and in the murders of six people in Gopibagh, said police.

EPZ Police Station Inspector (Investigation) Javed Mahmud, also investigation officer (IO) of the case, yesterday said Gaffer involved Mannan with JMB.

According to the FIR, Mannan, a civil staff at the Navy, threw two grenades upon the devotees at the mosque. Another militant Ramjan was also detained while he carried out another attack at a nearby mosque. Some 24 people were injured in the attacks.