Indian Mujahideen terror group, which claimed responsibility for the Varanasi blast on December seven, had started adopting the technique used by Abdul Karim 'Tunda' in late 1990s when acid was used to trigger an explosion.
The modus operandi might have been used in Varanasi blast which left two people including a child dead after a bomb exploded at a 'ghat' where the evening prayers were on, sources said. The authorities were not able to find any triggering mechanism used in the explosive material and an in-depth forensic examination was carried out.
The examination showed that the explosive material used in the Varanasi blast was ammonium nitrate with RDX used in little quantity to act as a booster.
The investigators were trying to ascertain the triggering mechanism used in detonating the bomb as this forms a crucial evidence in getting the tell-tale sign of terror group.
What came as a surprise to the forensic scientists as well as security agencies was the presence of sulphuric acid in a few places at the blast site, the sources said.
Joining the dots together, the security agencies were of the opinion that the terror module which carried out the blast at Varanasi had used a mixture of ammonium nitrate, RDX as a booster and sulphuric acid kept in a test tube, whose drops fell on the explosive material, as a triggering mechanism, the sources said.
The same kind of techniques were used by Lashker-e-Taiba's bomb expert Abdul Karim "Tunda" while carrying out a spate of bomb explosions in late 1990s in Uttar Pradesh. This also gave rise to suspicion that the Indian Mujahideen could have established a base in Bangladesh where Tunda had developed an association with a local students' group in late 1990s, the sources said.