Initially, investigators believed that the chemical composition of the bomb would be a mix of ammonium nitrate and fuel oil — a rather common explosives mix. However, laboratory analysis of the yellow explosive substance has now revealed the mix contained both ammonium nitrate and ammonium pictrate. Though fuel oil was not used as a composition, a third possible contaminant was probably used.

While no official report has yet been provided to investigators on the presence of ammonium pictrate — apart from oral communication — the finding assumes significance as it is classified as a military explosive.

“The tests have not been finalised but ammonium pictrate has been indicated. Though classified as a military explosive, it can be manufactured by someone with knowledge about the properties of picrate,” sources said.

According to Internet literature on explosives, ammonium pictrate or picric acid is used “as a military explosive because of its marked insensitiveness to shock and friction, which makes it well suited for use as a bursting charge in armor-piercing projectiles”. It has been at times preferred over TNT. According to bomb squad officials in Karnataka, only dog squads of the Army or the BSF are equipped to sniff this explosive out.

The polythene sack with the words ‘US Mail, US Postal service,’ that was used to cover the black trunk with 13 kg of explosives found at a roadside near Channapatna, has also raised questions. The bag, police say, is probably from the garment industry and had the words “100 per cent cotton denim fabric” printed on it. Excise and bale numbers are also embossed on it.

Investigations into the make of the microchips used as timer devices in Bangalore and in Surat have revealed that they were manufactured in the US and Thailand respectively.