Mumbai Police on Saturday claimed to have cracked the July 11 serial blasts in the city's commuter trains.

Addressing media persons, Mumbai Police Commissioner A N Roy said Pakistan's Inter Services Intelligence was the 'mastermind' behind the terror attacks carried out by Lashkar-e-Tayiba with help from Students of Islamic Movement of India activists.

Out of the 15 people arrested in connection with the blasts the 'direct role' of 12 people has been established, he said, adding that out of them, 11 are Pakistanis, who arrived in India in batches.

The conspiracy was hatched in Mumbai and adjoining areas after taking training in Bahawalpur in Pakistan, he said.

The Commissioner said the main players behind the blasts are Faizal Sheikh, Kamaluddin Ansari and Ehtasham Siddiqui, state general secretary of SIMI.

Two of the Pakistanis had come through Nepal reaching Mumbai around May 25. The second group had come via Bangladesh after spending some time in Kolkata while another separate group of four people had come through the Gujarat border.

Roy said they were kept in separate places in Mumbai. Faizal Sheikh, a local, had arranged rented houses for them one in Malad and four in Bandra in Mumbai.

About 15 to 20 kg RDX was used in the blasts, which was brought from Pakistan by one Ehsanullah. Ammonium nitrate, which was mixed with RDX, was provided by local groups. The bombs were put together in the Chembur area by one Mohammad Ali around July 8 to 10.

Roy said the bombs were kept in Faizal's home in Bandra from where they were transported to the Railway Stations in taxis. The bombs were kept in eight pressure cookers of five litre capacity, which were bought from two shops.

Each bomb contained two to 2.5 kg of RDX and 3.5 to four KG of ammonium nitrate. The pressure cookers were kept inside bags which were in turn camouflaged in things like newspapers and umbrellas, he said.

"It was a professional, precise and well-planned operation," he said.

The police commissioner said that the first clue received by the police was a phone call made from Navi Mumbai to a place along the Indo-Nepal border. Following this piece of information, police arrested prime suspect Kamaluddin Ansari from Madhubani in Bihar.

He said all the arrested were trained in Pakistan and knew how to dodge interrogators.

The narco-analysis tests conducted on the arrested persons also helped in tying several loose ends. Faizal Sheikh told police he had received around Rs 60 lakh from Pakistan during the last few years, Roy said, adding that police recovered 26,000 Riyals from his house, which he received from one Rizwan Devra, an ISI operative living in Saudi Arabia.