Investigators probing the Mumbai blasts believe the terrorists who planted the bombs may have used a "safe house" somewhere near the blast sites to manufacture the explosive - a deadly cocktail of ammonium nitrate and fuel oil.
According to sources, the investigators base their theory on the fact that the bombs were improvised explosive devices -- IEDs or home-made bombs - using ammonium nitrate and fuel oil, called ANFO.
The sources said the prepared IED is unstable and can go off by even a slight spark, therefore it would be difficult and dangerous to travel long distances with the bomb.
Even police patrolling at various places in and around Mumbai is so intense that the bombers may not have taken the risk of being caught with a bomb, they said.
Two of the three blast sites - Zaveri Bazaar and Opera House - are separated by a little over a kilometre, and Dadar, the third site, is about 12 km away.
That means, if a "safe house" was used, it could be close to or around Zaveri Bazaar or Opera House.
The sources also said the terrorists who planted bombs at three sites in Mumbai may not have been locals, an examination of the CCTV footage collected from the sites of Wednesday's blasts has indicated.
Investigation agencies are in possession of 11 CDs of CCTV footage recovered from the blast sites. The bombings left 19 dead and 129 injured.
The sources said that the Intelligence Bureau had received an input from Hyderabad in February that some Indian Mujahideen men were planning to strike.
But the information could not be followed up because the terrorists have stopped using cellphones and emails, they said and added that they may have met in person.
Earlier, union home secretary RK Singh said Friday that probe agencies are scanning 11 CDs of CCTV footage for clues into the Mumbai serial bombings.
The scooter on which one of the bombs was planted, has also been identified, he added.
"We also have identified the scooter in which one of the bombs was planted. There are various leads based on our previous databases, various people are being questioned, so investigation is going on," he added.