Some policemen took the bomb inside Pallabi Police Station to stage a bomb-recovery and frame someone in a case.
But due to clumsy handling, it exploded in an officer's room, injuring four policemen and a civilian at the station on July 29.
Officials familiar with the matter said a section of crooked policemen "collected" the bomb which was fitted inside a weighing machine from the associates of a local political leader. The aim was to frame the associates of another leader.
And they received hefty sums to pull off the staged "bomb recovery", the officials said, requesting not to be named.
After the explosion, Dhaka Metropolitan Police formed a three-member committee led by Joint Commissioner (operations) Monir Hossain to investigate the incident.
The committee concluded that the policemen posted at Pallabi Police Station were solely responsible.
The officers were not supposed to take the bomb inside the police station and they did not inform the bomb disposal unit on time, the investigators said after a weeklong probe.
The investigators also said the officer-in-charge of Pallabi Police Station and deputy commissioner of Mirpur Division failed to give proper instructions regarding the situation.
Joint Commissioner Monir told The Daily Star that the probe report has been submitted before the DMP commissioner.
Contacted, DMP Commissioner Shafiqul Islam told The Daily Star yesterday, "The bomb was not handled properly at the police station and they acted unprofessionally."
DMP's Mirpur division DC Mostaq Ahmed, Additional DC (Pallabi) Mizanur Rahman, Assistant Commissioner (Pallabi) Firoz Kawsar, Pallabi Police Station Officer-in-charge Nazrul Islam, Inspector (investigation) Abdul Mabud and Inspector (operation) Emranul Islam have been transferred.
DMP boss Shafiqul Islam said the transfers were not punitive.
"I need to go through the probe report and separate punishment awaits whoever is responsible," he added.
SOURCE OF BOMB
Investigators said they identified two suspects named Jamil and Mamun. They are brothers and are believed to be hiding somewhere in India and Nepal.
"The two brothers took a contract to kill either Pallabi Jubo League General Secretary Juwel Rana or Councillor of Ward-6 Taizul Islam Bappi," said a police official, involved with the investigation.
Police then started investigation and arrested Shahidul Islam, Mosharraf Hossain and Rafiqul Islam on July 28 and July 29. They are associates of the suspect Jamil, sources said.
"After the arrests of these three, Councilor Bappi made a deal with a section of Mirpur division police, offering around Tk 2 crore, so that the arrestees are produced as members of Juwel Rana's gang," said the official.
Both Bappi and Juwel Rana are close aides of ruling party lawmaker Elias Mollah, he said.
"But, Bappi wanted to get rid of Juwel Rana as he was trying to run for the post of the councillor. Besides, Bappi wanted to establish supremacy and take control of Pallabi areas. For that reason, Bappi wanted to drag Jewel's name into criminal activities," according to police sources.
As per the deal, a police team took the arrestees near Kalshi graveyard area and collected a firearm from Bappi. Police also collected the bomb and some bullets from his associates.
The officers then took the bomb, firearm and ammo to the police station. They were arranging them at an officer's room when the bomb went off, said multiple officials involved with the investigation.
A high official of DMP, requesting anonymity, said the six top officials of Mirpur division were transferred right after the investigators learned about the plot and the deal.
The Special Action Group (SAG) of Counter Terrorism and Transnational Crime (CTTC) unit is now investigating the cases filed after the blast.
SAG Deputy Commissioner Abdul Mannan told The Daily Star yesterday that some issues had come forward and they found a number of people's involvement.
But he refused to disclose further details for the sake of the investigation. "We are now analysing the facts."
Contacted, Councilor Bappi said he had a very good relationship with Juwel.
"I am not involved in any kind of plots. Some people might be spreading rumors against me to smear my image in politics and destroy my relationship with Juwel," he said.
Bappi further said that he has come to know that someone wanted to kill both of them.
Without disclosing the name of who he thought might have wanted him dead, Bappi said, "Police are investigating the case and we are expecting a fair investigation."
The Daily Star also tried to contact Juwel, but his phone was unreachable.
Regarding the person, who might have hired Jamil and Mamun to kill someone in the first place, a high official of DMP, involved with the findings, said their investigation was stuck because they were out of the country.
"We are now trying to find his associates and collect information about their activities. Once they get arrested, we would be able to say further about the matter," the official added, requesting anonymity.