Scale of blast hints at military-grade explosives, clarity likely in a few days

NEW DELHI: There is mystery over the kind of explosive or chemical used in Monday's blast with the forensics and NSG's explosive wing yet to submit their report. However, prima facie assessment points towards the use of military-grade explosives, given the scale of damages and casualties.

Sources said investigators on Tuesday sought an opinion from forensics on whether PETN, semtex or RDX could have been used. The initial assessment also points towards the possible use of ammonium nitrate fuel oil and detonators. The nitrate oil is used both industrially and in improvised explosive devices due to its easy availability and cost-effectiveness. The forensic teams have collected roughly 42 exhibits from the site and put them through lab testing.

The exhibits include the i20 car's tyres, chassis, CGC cylinder and parts of the bonnet, besides other residues and traces of powder found at the site. They will help determine the types of high-exposure materials used in the explosion. Pentaerythritol tetranitrate, or PETN, is a major ingredient of semtex and belongs to the same chemical family as nitroglycerin. One of the most powerful explosives, it is preferred by terrorists because its colourless crystals are hard to detect.

A very small quantity of this explosive is required to carry out a blast in comparison to other chemicals. For example, the explosive substance used in the foiled transatlantic air freight bomb plot in 2010 was concealed in a printer cartridge. "PETN is quite stable and detonated either by heat or a shock wave. A car can be destroyed in around 100 grams. Most importantly, neither PETN nor semtex inherently require pellets or shrapnel to act as an explosive and the material itself is enough to cause immense damage through detonation," said an officer. On the other hand, ammonium nitrate fuel oil produces a supersonic shock wave rather than a subsonic burn, which means energy is released at an extremely high rate, creating a powerful shock effect. In the Delhi High Court blast of 2011, the initial opinion of forensic experts, including that of National Security Guard, was that a mix of ammonium nitrate with PETN was used. However, the home ministry had said later that RDX was used in it. Sources said there would be complete clarity on the explosive used in Monday's blast in a few days. Investigators suspect a proper detonation system was in place to blow the car, which was rigged with high-end explosives. A watch and its small button batteries may have been used for building the circuit and triggering the bomb.