Police have ruled out any militant involvement in Wednesday's blast in Pallabi Police Station in the capital.
Investigators and Bomb Disposal Unit officials said the bomb was not similar to those used by militants in recent years.
Home Minister Asaduzzaman Khan Kamal also said there was no connection between extremist outfits and the explosion.
Just around a week ago, on last Friday and Saturday, an improvised explosive device (IED) went off in Paltan, and a grenade-like object was found at Gulistan intersection.
Talking about Wednesday's blast, an explosive expert and former official of the Department of Explosives told The Business Standard that it is hard to believe that local criminal gangs are using explosives instead of modern weapons and firearms.
"The explosives are riskier and prone to be detected by police at any time. Instead, using guns or firearms is easier to create panic," he said.
The Counter Terrorism and Transnational Crime (CTTC) unit of the Dhaka Metropolitan Police (DMP) will investigate the incident.
"The Bomb Disposal Unit will carry out a thorough probe," Rahmatullah Chowdhury, additional deputy commissioner of the unit, told The Business Standard.
"We visited the spot and got to know the type of bomb. It does not seem to be an IED bomb of the type used in the last couple of years."
"Looking at the bomb parts, we think a local criminal gang made it."
Two CTTC high officials and officials of the detective branch of the DMP told The Business Standard that members of the underworld gang "Shahadat Bahini" planned to kill a councillor of the Dhaka North City Corporation.
"They were planning to destabilise the political situation in the Mirpur area, and procured these explosives to blast them during a political assembly of the councillor," DMP Additional Commissioner Krishna Pada Roy said.
"They got the call from Shahadat and were planning to pull the job off, but Mirpur police somehow managed to detain them in a short time. Unexpectedly, one of the bombs exploded inside the police station," an ADC of Detective Branch Mirpur Division said.
A three-member probe committee has been formed to investigate the blast, and has been asked to submit a report within three days, said DMP Deputy Commissioner (media) Md Walid Hossain. DMP Joint Police Commissioner (crime) Md Shah Abid Hossain leads the committee.
On July 19, the police headquarters issued an alert regarding possible militant attacks ahead of Eid-Ul-Adha and the month of August when the National Mourning Day is observed.
How the bomb exploded
Around 7am on Wednesday, a bomb suddenly exploded on the first floor of the rented premises of Pallabi Police Station.
"The blast smashed most of the windows of the rooms. Pieces of glass were scattered on the road outside the police station. The sub-inspectors' room turned into a battlefield within seconds," said Riaz, a civilian who was injured in the blast.
Five people, including four policemen and Riaz, were injured. The four others are Inspector (operation) Imran, Sub-Inspector Sajib, and provisional sub-inspectors Rumi and Ankush.
Krishna Pada Roy said, "Shortly after the blast, we found two more explosives and were able to successfully defuse them. Those were in a box that resembled a weight machine."
Pallabi Police Station Officer-in-Charge Nazrul Islam said a police team had confiscated bombs during an operation in the Kalshi area around 2am on Wednesday and tried to defuse them at the police station.
"We informed the Bomb Disposal Unit to neutralise them. However, one of the bombs exploded in the morning before the unit reached the police station," he added.
Krishna said, "Three criminals of a local gang were arrested during the operation in the Kalshi graveyard area. Two foreign pistols, three bomb-like objects and a weight machine were seized from their possession."
"They planned to commit a crime. We initially learned that it could be a murder, robbery or property-related crime," he said.
No involvement of religious militant groups has been found so far, he added.
The arrestees – Rafiqul Islam, Shahidul and Musharraf – were taken to the CTTC office for further interrogation.
Two days ago, top police officials of different units were instructed to remain alert as police suspected extremists' links to the explosion in Paltan and the bomb-like item found in Gulistan.
Police headquarters issued a letter regarding a possible attack by members of the so-called Islamic State just ahead of Eid in the name of their so-called mission "Bengal Uliyah" or "Bengal Khilafat," and alerted all the district superintendents of police and other respective units.