A forensic expert collects samples from the blast site. Picture by Sanat Kumar Sinha

The shell that exploded in Chitpur on Sunday was stuffed with nitro-cellulose and nitro-glycerine along with metal-salts like sodium chloride and sodium sulphate and the moment an iron rod touched the contents, it triggered a chemical reaction leading to the blast, state forensic experts concluded on Monday.

“When one of the victims tried to open the shell with an iron rod, it pierced through the solid metal casing made of copper and zinc alloy and reached the chemicals inside, resulting in a chemical reaction,” said an expert of the state forensic laboratory.

Experts from both the state and the central forensic science laboratory visited the blast site at Chitpur’s Jyotinagar Colony on Monday, collecting “post-blast residue” and combing the area for definitive clues. Both agencies said they would need a fortnight to come out with a conclusive report.

“Apart from carrying out laboratory analysis of the samples, we would also have to examine the post-mortem reports of the victims before we arrive at a conclusion. This is a shell for sure, but it is difficult to tell the age,” said Dhurjoti Sengupta, the director of the state forensic laboratory.

Police, however, said they are yet to establish if the shell was manufactured by the Ordinance Factory Board and are trying to find out how such a shell reached the shores of the Hooghly.

“We are working on the case and only after the shell’s make has been confirmed would we start a dialogue with the authorities concerned. The two youths who picked up the shell have died and this is hindering the probe,” said Jawed Shamim, the city detective chief.

Till late on Monday, post-mortem reports of two of the four victims — Ramzan Molla and Dedar Seikh — revealed that they died of a “shell blast”. The report of the other two are yet to arrive.

“There are blackening, singeing and burn marks on the bodies, suggesting that the duo died of a shell blast,” an officer of the detective department said.

On Monday, the city continued to be on the edge with repeated bomb scares. In the morning, cops and the bomb squad rushed to inspect two tin cans that were found tied with a rope under the Taratalla flyover, while in the afternoon, panic spread in a Girish Park bank over an unattended tiffin box. Both turned out to be false alarms.