"We have questioned 250 to 300 people as part of the investigations in two days," said A.N. Roy, Mumbai's police chief. "Around 20 people have been detained at various places."

Most of the detentions were made overnight in Malwani, a northeastern suburb of Mumbai, said police Inspector S. Goshal. He said nobody has been formally arrested or charged, and the detainees were rounded up only for questioning to help with the investigations into Tuesday's bombings.

At the same time, investigators picked through the mangled ruins of train cars, placing evidence in blue plastic bags beneath an overcast monsoon sky.

"We are just trying to establish what kind of explosives were used and where exactly the bombs were placed, but it appears they were kept in the luggage racks," said police inspector Yeshwant Patil.

Pasricha said investigators were looking into a possible link with Lashkar-e-Tayyaba.

"It is difficult to say definitely as this stage, but Lashkar-e-Tayyaba can be involved going by the style of attack," said Pasricha.

But other Indian officials, speaking on the condition of anonymity, cautioned that it was too early to point fingers at a specific group.

Lashkar has denied any role in what it called "inhuman and barbaric acts".