Forty-eight hours of rainfall may have saved dozens of lives by degrading the explosive material used in the six improvised explosive devices (IEDs) planted on a crowded road in Pune on Wednesday, a forensic expert associated with the investigation told The Hindu.

Though the detonators went off, he said, humidity absorbed by the explosive material (ammonium nitrate and fuel oil mix) prevented the bombs from exploding.

Pune registered 83.9 millimetres of rainfall on Wednesday and 18.1 mm the previous day, with humidity hovering at over 90 per cent, meteorological records state. In April 2010, two bombs planted outside the M. Chinnaswamy Stadium in Bangalore fizzled out in near-identical circumstances, while a third failed to detonate. Bombs planted in Bangalore in 2008 and a car-bomb parked outside the Delhi High Court last summer also fizzled out because of humidity-related degeneration in the ammonium-nitrate based explosive.

Sources in the Mumbai Police said unexploded bombs found in Pune on Wednesday and Thursday bore some similarities with the devices used by the Indian Mujahideen, which has carried out multiple terror strikes in which hundreds of people have been killed since 2005.

However, a senior police officer said it would be “wrong to attribute responsibility to a group on the basis of the bomb-type alone, because these devices are easy to fabricate.”

Though classified as a low explosive, the ammonium nitrate-fuel oil mix has been used to lethal effect by insurgents and terror groups around the world —notably in 1995, when a car-bomb in Oklahoma killed 168.

Ammonium nitrate is widely used in India as a fertilizer; its sale is largely unregulated.

In its first three strikes — the bombing of Varanasi’s Ashwamedh Ghat in February 2005, the July 2005 explosion on the Sharamjeevi Express and serial explosions at a Delhi market in October 2005 — the Indian Mujahideen used military grade explosive pentaerythritol tetranitrate or PETN.

However, in its subsequent operations, among them the attacks on the Mumbai’s mass transit system in July 2006 and the bombing of the German Bakery in Pune in February 2010, the Indian Mujahideen used ANFO devices.