The CID on Wednesday started investigating how the bomb that killed a child in Kazipara and injured several people on Tuesday found its way to the ground floor of a four-storey building.

It appears that the “socket bomb” was kept in a jute bag, along with other bags containing fruits, and it exploded because of sudden pressure, a CID officer who was part of a team that inspected the area said.

“It’s the first day and investigations are in the initial stages,” another officer of the visiting team said. “Primarily it doesn’t seem any terror outfit was involved.

“If any banned outfit was involved, their members would have chosen a more crowded area instead of a lane that would be much less crowded, especially on a holiday.”

Ajit Halder, the fruit-seller, who had set up his shop in front of the building and was bang in the middle of the explosion is admitted to RG Kar Medical College and Hospital.

The CID would speak to Halder as soon as doctors give the permission, the officer said.

“The bomb was placed behind the fruit-seller and that explains his hip injuries,” the officer said.

“The shop selling roti and aloo sabzi and the sweet shop had opened early. If a the bomb had been in the open, someone would surely have spotted it while cleaning the area. But that didn’t happen... so, we want to know how it landed in that place.”

Investigators are trying to identify the various explosives packed in the socket, along with ammonium nitrate, that caused such an impact, the CID officer said.

The impact was felt almost 29ft from the seat of explosion, he said.

The intensity left the windowpanes on the fourth-floor of the building shattered.

A team of officers visited all the floors of the four-storey building, which took the maximum brunt of the blast.

The officers spoke with residents, trying to gauge how they felt when the explosion occurred.

The team visited the sweet shop on the ground floor to find the glass shelf shattered.

The shop owner couldn’t be found.

In the absence of any CCTV cameras along Kazipara Road, the CID team made a sketch of the area, showing the blast site, the adjoining shops and their sizes, the number of buildings in the area and the blast-hit spots.

After speaking to residents, some of whom had witnessed the explosion, officers drew up a preliminary list of those who would be questioned.

A team from the Central Forensic Science Laboratory collected samples from the spot after the CID team was through with their work.

The forensic team visited areas around the blast site and collected samples.

On Tuesday, officers of Dum Dum police station had started a murder case.

On Wednesday evening, the case diary was handed over to the CID.

“The case has been prepared under sections 302 (murder), 307 (attempt to murder), 324 (voluntarily causing hurt by dangerous weapons) and 120B (criminal conspiracy) of the IPC, along with sections of the explosive substances act,” an officer of Dum Dum police station said.

“We handed over the case details to the CID, along with all items seized from the area,” the officer said.