Teams from the state CID and forensic department started their investigation into the explosion at an illegal firecracker factory in Egra area of West Bengal's Purba Medinipur district on Wednesday, a senior officer said.
Forensic experts started collecting samples from the now collapsed house at Khadikul village which housed the illegal firecracker unit and its surroundings, he said.
The CID officers will speak to witnesses as well as with family members of those killed in the blast, the officer said. A huge posse of local police have been deployed in the village near the Odisha border.
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The district police have lodged an FIR in connection with the explosion that took place on Tuesday in which nine people were killed and several others injured.
"Our officers are at the spot and they have started their investigation. Forensic experts are also there. The samples collected from the spot will be sent for examination to identify the materials used at the factory to prepare firecrackers," the police officer told PTI.
The condition of the five people injured in the last blast is stated to be "critical", one doctor of the hospital, where they are undergoing treatment, said.
"One woman is extremely critical and the condition of the other four can only be described as 'not out of danger'," the doctor said.
Meanwhile, BJP leader Suvendu Adhikari reached Khadikul village on Wednesday morning. He spoke to villagers there and tried to take a look at the house where the firecracker factory was housed, from the top of the terrace of a neighbouring building.
Adhikari who hails from Purba Medinipur district repeated his demand for an NIA probe into the explosion and said that he and his party workers would hold a rally to demand a probe by the central agency.
"We want an NIA probe into the matter. We want the truth to come out and the culprit to be punished," Adhikari, the Leader of the Opposition in West Bengal Assembly said.
West Bengal Chief Minister Mamata Banerjee on Tuesday said that though the state CID has been entrusted with the duty to probe the explosion, she has no objection if the NIA starts investigating the matter.
"We do not have any objection (to it). It is the district of their (BJP) leaders, so they have some formula. Let the investigators find out," Banerjee had told reporters.