India remains on high alert after the terror attacks on Wednesday that killed 18 people and wounded 133 others in Mumbai.
Prime minister Manmohan Singh has vowed to bring to justice those behind the triple bombings in the evening rush hour.
''The terrorists had the advantage of surprise,'' he said at a hospital after meeting some of the injured.
''This time there was no advance indication. Now our task is to find out who the culprits are and how we can work together to bring them to justice.''
The ABC reports police are questioning members of a home-grown Islamist group.
The Indian Mujahideen has been accused of ties to Pakistani groups involved in attacks in Indian Kashmir.
However, home minister Palaniappan Chidambaram said it was too early to point the finger at a particular group.
India's financial capital was previously attacked in 2008, when 166 people were killed.