As forensic scientists begin poring over the wreckage of Tuesday’s seven train bombings in Mumbai, India, experts say the defusing of an eighth bomb could prove crucial to the investigation.

"It's enormously important to have discovered and defused one of the bombs," says Sidney Alford, a UK-based bomb disposal expert. "It lets you make some pretty critical deductions. You can work out the source of the explosives and perhaps the detonators. You may even get fingerprints or DNA."

In the space of 11 minutes on Tuesday 11 July, seven synchronised bombs blew apart several commuter trains and train platforms, killing 190 and injuring more than 700 people. Rush-hour traffic, a shortage of ambulances and monsoon rains combined to impede rescue efforts. "It took hours getting the bodies inside the hospitals," witness Bunty Jain told AFP. No terrorist group has yet claimed responsibility for the bombings.

Part of the forensic operation will be directed towards identifying the explosive materials used and tracing other components. This could help police identify those responsible.

Mass spectrometry

"From the unexploded bomb, they will be able to identify the substance the main explosive charge is made from," Alford told New Scientist. "They can then use mass spectrometry to identify the sources of the individual ingredients of the explosive."

Colin King, editor of the military journal Jane's Explosive Ordnance Disposal agrees that the unexploded device be vital. "It is hugely significant in terms of saving time," he says. "It could save investigators weeks of painstaking reconstruction of evidence from the disaster scenes."

According to the Mumbai police department, the bombs were made using a "high grade explosive material", rather than a readily available material such as fertiliser (ammonium nitrate).

Improvised devices

The last time Mumbai suffered a terrorist attack, in August 2003, the high-grade explosive RDX was used. This is a commonplace military explosive used in artillery shells and is used by Iraqi insurgents to make Improvised Explosive Devices, often planted at roadsides.

If RDX was used to make the Tuesday bombs, it could therefore be difficult to trace to any particular group. However, analysis of other bomb components could help narrow things down.

"The detonator could be home made but it is much more likely to be factory-made, with a batch number on it, so it may be traceable to a quarry, or coal mine," Alford says. Electronics components, such as a timing or radio transmitting device, could also be "eminently traceable", he adds.