The police and Rapid Action Battalion (Rab) rushed to the spot, cordoned off and searched the area for about half an hour. They recovered some scattered splinters from the ground.

Eyewitnesses said the bomb was thrown through a second-floor window of the building at four policemen on routine security duty in the BNP office on the first floor, but missed the target and exploded outside. They said the bomber made good his escape after the attack.

Khulna PS Officer-in-Charge Mohammad Iqbal also confirmed that the bomb was hurled from the second floor, from where the police picked up three people for questioning. The floor was later sealed off.

The arrestees are Hafez Nasiruddin, 24, son of late Momin Molla of Katenga under Terokheda upazila, M Abul Hossain, 22, son of Shahar Munshi of Dearha under Rupsha upazila, and Tajul Islam, 25, son of Kazi Azizul Islam of Musalman Para under Khulna upazila in the district.

Nasiruddin works as a salesman at a cosmetics shop and Abul Hossain at a cloth shop. Both the shops are on Clay Road in the city, Khulna police said.

The police also recovered electrical wires and plugs and short iron bars from their second-floor room. The law enforcers were interrogating them at Khulna PS until filing of this report late last night.

In immediate reactions, Khulna BNP leaders expressed grave concerns at the incident and demanded a thorough investigation into it.

Tight security has been arranged for the BNP office, where no-one is now allowed to enter without thorough search.