If so, perpetrators had access to military-grade explosive

Forensic scientists of the National Security Guard (NSG) believe that the lethal high explosive PETN (pentaerythritol tetranitrate) was used in Wednesday's blast in the Delhi High Court complex, sources in the investigation told The Hindu.

The use of PETN raises the prospect that the perpetrators had access to military-grade explosives — either from terrorist groups operating in States like Jammu and Kashmir or from a military source.

Except the 2010 bombing of the German Bakery in Pune, all post-26/11 urban terror attacks have used ammonium nitrate — easily available for use in mining and as a fertilizer.

Investigators, however, urged caution, saying the NSG's finding needed to be corroborated by more thorough tests now being conducted at the Central Forensic Sciences Laboratory.

Two separate sources in the investigation said the pattern of the Delhi High Court strike did not appear to indicate the use of plastic explosives.

Early findings of Indian bomb-tests have often been disproved by subsequent investigation. The 2006 attacks on Mumbai's train system were thought to have used RDX, but later the chemical was found to be ammonium nitrate.

PETN is used by both militaries and terrorist groups. In 2001, al-Qaeda operative Richard Reid attempted to bring down a transatlantic flight using PETN hidden in his shoes. Eight years later, the Nigerian al-Qaeda operative, Umar Farooq Abdulmutallab, made a similar attempt, hiding 80 gm. of PETN inside his underwear.

Al-Qaeda also used PETN for an attempt to bring down cargo planes last year using bombs hidden in inkjet printer cartridges.

Difficult to detect

The explosive is favoured by terrorists because sniffer dogs and machines find it difficult to detect it.

Maharashtra investigators made an arrest in the Pune blast, and are seeking six fugitives, including the men they claim were responsible for fabricating and planting explosive devices. There has so far been no account of where the explosive material used in the bombing was obtained. Intelligence sources said RDX might have been sourced from terrorists in Jammu and Kashmir, but no hard evidence had emerged.

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