Amid an existing security alert, a bomb fitted inside a "weighing machine", which police seized, exploded inside Pallabi Police Station yesterday morning, injuring four policemen and a support staff.

Police ruled out the involvement of any militant outfit, but US-based terrorism analyst SITE Intelligence Group in a tweet last night said IS claimed credit for the blast.

SITE co-founder Rita Katz tweeted, "#ISIS claims attack in #Bangladesh… Marks the group's 1st attack in the city since Aug. 2019 & comes amid an attack surge as part of the 4th wave of its 'Battle of Attrition' campaign, in lead up to #EidAlAdha."

Asked about this, Saiful Islam, deputy commissioner of Counter Terrorism and Transnational Crime (CTTC) unit of Dhaka Metropolitan Police, said the claim was baseless.

"ISIS has made this type of fake claims before. This one is nothing but part of that process. We have so far found criminal intentions behind the incidents and we are analysing the motive. We are investigating the incident to check further," he told The Daily Star.

Another CTTC officer said ISIS simply tried to take the opportunity because the blast was widely covered by the media.

There was a blast in Paltan on Friday, but no one claimed the responsibility as it got poor media coverage, he said.

There were no casualties in Friday's blast.

Walid Hossain, DMP deputy commissioner (media), told reporters that there was information that the bomb was intended to be used against a local political leader.

He did not say which politician was the possible target.

"The issue arose from political rivalry between local leaders. An investigation is ongoing and things will be clear once it is complete," he said, adding that police had information that three suspects, who were arrested in possession of firearms and the "weighing machine" yesterday, were "hitmen".

DMP Additional Commissioner (Crime) Krishna Pada Roy said Pallabi police raided Kalshi area around 2:00am and arrested the suspects after learning about a possible "criminal act" by a gang.

The police team found two pistols, four bullets and something appearing to be a weighing machine in their possession, he told reporters in front of Pallabi Police Station.

Being suspicious as to why the suspects would carry a weighing machine, the officers called DMP's bomb disposal unit. After going there and examining the device, the unit recommended further inspection by a more experienced squad.

But it exploded around 6:00am before the special team arrived, Krishna added.

Duty officer Aklima Akhter said the explosion was in the room of an inspector on the first floor of the five-storey building.

Inspector (operations) Imranul Islam, 48, and sub-inspectors Sajib Khan, 30, Angkush Kumar, 28, and Rumi Haider, 28, and support staff Mohammad Riaz, 28, were injured.

Rumi and Riaz were admitted to Dhaka Medical College Hospital and Angkush was taken to the National Eye Institute.

Riaz's left wrist was blown off and his right hand was wounded. He also suffered injuries to his stomach, said Resident Surgeon Mohammad Alauddin of DMCH.

The explosion happened at a time when there was intelligence on possible militant attacks by the IS-inspired "Neo-JMB" during Eid-ul-Azha.

The Police Headquarters had earlier alerted its units to a possible attack.

Krishna Pada Roy, however, said there was no evidence linking the incident to militancy.

"The arrestees are members of a criminal group. We are interrogating them," he said.

The arrestees, Shahidul Islam, 23, Mosharraf Hossain, 26, and Rafiqul Islam, 40, are members of a gang called Shahadat Bahini, said a high official of the DMP.

Locals said the explosion rocked the neighbourhood and caused panic. The blast shattered most of the glass windows on the first floor of the building.

Another high official, requesting anonymity, told this correspondent that the bomb exploded when one of the policemen of the station tried to check it out.

Additional Deputy Commissioner Rahmatullah Chowdhury, in-charge of the CTTC's bomb disposal squad, said the bomb was powerful and was made by professionals.

"Inside the weighing machine, there were three pipes. One of the pipes had exploded before the bomb disposal team reached. Later, we defused the two other pipes," he added.

"Militant groups do not use this type of explosive. We usually see criminal groups using these explosives," he said.

Earlier on February 28, an improvised explosive device  went off at a traffic police box in Chattogram's Sholoshahar, injuring four people, including a child.

In September last year, a bomb exploded at an Awami League office in Khulna city. Before that, two people, including a female police officer, were injured when a bomb went off near a police pickup in the capital's Malibagh in May. Police cla