Police sources said Mamun alias Killer Mamun actively took part in the killing of deputy assistant director (DAD) Humayun Kabir and constable Phool Mia of Rab-11 at a brick kiln at Salehpur.

Investigators, meanwhile, said they have identified the mastermind and all those who took part in the killing. "Capturing them is now a matter of time," an investigator said, seeking anonymity.

The Rab identified Mamun, 30, as a member of the killing squad of local criminal gang Gangchil that operates in Aminbazar area.

The law enforcers said Mamun, who has five criminal cases including four filed for murders, was arrested in Krishnanagar village in Savar at 5:30am Monday.

During interrogation at the Rab-4 office, Mamun disclosed detailed information about Gangchil group's secretly cached firearms, names and whereabouts of its members, especially those in the killing squad of the two Rab personnel.

A special team of Rab-4 took Mamun to Baradeshi area beside the Dhaka-Aricha Highway at 3:45am yesterday to recover illegal firearms of Gangchil group, according to a Rab press release. When the Rab men reached an open space behind Chistia Filling Station, members of the gang opened fire on the Rab team, prompting the law enforcers to retaliate.

Mamun fell in the line of fire and was found lying on the ground bullet-hit when the firing came to a stop.

The Rab press release said doctors at Upazila Health Complex declared him dead when he was taken there.

The Rab claimed they recovered a .22 bore revolver and five bullets from the spot.

Talking about the investigation, an investigator said they have all information necessary to track down the killers.

"The whole mystery is now resolved and we will make public everything after capturing the killers," he said, declining to disclose the names of the killers and their mastermind for the sake of investigation.

Unknown killers took Rab intelligence wing personnel DAD Humayun and constable Phool Mia to a hut at Tara Mia's brickfield at Salehpur of Aminbazar in the outskirt of the capital and shot in their heads around 3:30pm Saturday.