One of the 11 Pakistanis involved in the July 11 serial train blasts had come here exclusively to offer his expertise for making bombs which were subsequently used to strike terror in the local trains, investigations by the Anti-Terrorism Squad reveal.
The bomb expert assembled the bombs using Research Developed Explosive (RDX), ammonium nitrate and quartz timer in pressure cookers at the residence of accused Mohammed Ali in Shivaji Nagar slum in north east Mumbai, ATS sources said.
Pressure cookers were used strategically in the blasts as they cause high intensity explosions and are compact in size, they said.
The core of the cookers comprised a lump of RDX placed in the centre surrounded by ammonium nitrate. A detonator comprising a quartz timer was placed over the explosives to trigger the explosions, sources said.
Two to two-and-a-half kilogram of RDX, four to five kilogram of ammonium nitrate and a quartz timer were kept in five-litre pressure cookers to cause the seven explosions, which killed nearly 200 persons and injured over 600, according to Mumbai police commissioner AN Roy.
"The compression of all these materials into a pressure cooker gave a high intensity to the explosions," the source said.