Sources in the forensic team that analysed the samples said the same combination—packed inside pressure cookers—was used in the Varanasi blasts on March 7, 2006.

“While a detailed forensic report may be submitted to us by tomorrow, it has been authenticated that samples of RDX, ammonium nitrate and fuel oil were found from post-explosion debris (in the seven trains). Forensic experts from Hyderabad, Kalina and NSG have been working on it,” said Joint Commissioner of Police, ATS, K P Raghuvanshi.

“They used timers to set off the explosions. Maybe, they used RDX and ammonium nitrate together to intensify the blasts. This combination has rarely been used,” he said.

Meanwhile, an ATS team has left for Tripura where 11 Thane residents were arrested last Thursday not far from the Bangladesh border. “Our team will question the 11 men there. If they find any blast link there, the men will be brought to Mumbai for further interrogation,” Raghuvanshi said.

Sources in the forensic department said the bomb-makers used ammonium nitrate to ‘desensitise’ the “highly explosive and sensitive RDX powder to a degree” and ensure that the bombs exploded at precise timings. Analysts also found that common integrated circuits (ICs) were used to detonate the bombs.

The sources added that besides Varanasi, the other instance of this combination being used in a major bombing in recent times was on October 12, 2002 in Bali, where 202 persons died. While ammonium nitrate and RDX are a rare combination in bomb-making, a mixture of ammonium nitrate and fuel oil is a common blasting agent with the acronym ANFO, and is widely used by miners across the world.

Incidentally, ANFO was the explosive used in the attack on the Alfred Murrah Federal Building in Oklahoma city on April 19, 1995, in which 168 persons died.

Fuel oil is also known as reduced crude, which is the residual matter after refining gasoline, naphtha and kerosene from crude petroleum.