Forensic science experts working on the case told rediff.com that each bomb weighed approximately 6 kg and comprised ammonium nitrate, engine oil, gelatin sticks, concrete shrapnel, pebbles, nuts and bolts. This in turn was connected to an embedded chip which acted as a timer device and helped detonate the bomb.

The use of ammonium nitrate is not new in terror operations. The attack in Oklahoma City, USA, in 1996 and the Bali, Indonesia, bombings of October 2002 involved the use of ammonium nitrate.

Forensic experts say ammonium nitrate is converted into a powerful explosive when it is mixed with fuel as was done in the serial blasts in Bangalore and Ahmedabad. Ammonium nitrate produces gas in quick time. As the gas expands it causes an explosion. Ammonium nitrate acts as an accelerant which in turn speeds up the rate at which the fuel burns, producing a huge explosion.

Ammonium nitrate is easily available in the market. Transportation of ammonium nitrate would not raise suspicion -- unlike RDX -- as the chemical is not a banned substance. With security on the India-Pakistan border becoming almost impregnable, the transportation of RDX has become next to impossible.

Terrorists in India used Neogel-90 for the first time in the Hyderabad blasts last year. This substance used in quarrying activities is easily available in the mining sector. Following the Hyderabad and Jaipur blasts, curbs have been placed on the supply of Neogel-90, hence the terrorists may have decided to use ammonium nitrate.

If you look at the Bangalore and Ahmedabad blasts, none of the substances used is banned. Be it ammonium nitrate, embedded chips, fuel or nuts and bolts, all are freely available in the market. No suspicion would be raised when buying these substances.

While the bombs in Bangalore were packed in concrete in the shape of a flower pot, in Ahmedabad the bombs were placed in tiffin boxes. Experts say packing bombs in tight containers increases the impact. The more compact the packing, the greater the impact, the experts say.

Over the years the terrorists have used concrete blocks, tiffin boxes and pipes to pack the bombs. According to the experts, bombs packed in tiffin boxes and pipes have proven to be the most destructive to life and property.

While making these bombs the terrorists usually pack it with pellets. In the serial blasts in Bangalore the bombs were packed with nuts and bolts. Another aspect to the bombs used in both blasts was the use of embedded micro chips. Experts point out that embedded chips produce a fuse pulse that triggers a detonation. It can be programmed like a digital clock.

Experts probing the bomb that was defused in Bangalore on Saturday morning say it was programmed to go off nearly an hour later. It was defused in the nick of time.