New Delhi: A home-grown Islamist terrorist group, the Indian Mujahideen (IM), is suspected to be behind the three moderate to high intensity explosions in Mumbai on Wednesday which Home Minister P Chidambaram said were "coordinated attack by terrorists".

The terror attack, which claimed nearly 20 lives, comes days after two suspected Indian Mujahideen operatives, who provided vehicles used in the 2008 serial blasts in Gujarat that killed 56 people, were arrested from a Mumbai suburb by the Maharashtra Anti-Terror Squad (ATS).

Home Minister P Chidambaram said the three explosions -- at Opera House area in south Mumbai, Zaveri Bazar and Dadar West -- took place between 6.45 pm and 7 pm and "therefore we infer from this that this was a coordinated attack by terrorists".

The bombings at crowded places in Mumbai, which has turned out to be a soft target for terrorists, bear a "signature" of what the IM had used in carrying out earlier blasts in Bangalore, Ahmedabad and New Delhi in 2008, Varanasi and Pune in 2010, intelligence sources said.

However, they said it was just a "preliminary observation" as a detailed inquiry by a joint team of premier terror probe agency National Investigation Agency (NIA) and the ATS would reveal who was behind the blasts.

The sources said that the arrested IM suspects - 32-year-old Mohammed Mobin and his cousin 28-year-old Ayub Raja arrested with arms and ammunition on July 6 - were being questioned afresh if they could give some information on the blasts.

The sources said that material evidence collected from the blast site point that RDX and ammonium nitrate were used in the explosions.

"The pattern of the blast point towards the IM's modus operandi," sources said on condition of anonymity as they are not supposed to speak to the media during the course of the investigation.

The IM had owned up to a series of blasts in various cities across the country in 2008. It is believed to be a shadow organisation of the banned Students Islamic Movement of India (SIMI) and the Pakistan-based Lashkar-e-Taiba.

The sources said the explosives were "hidden", the way IM operatives did in 2008. These are all trademarks of the IM and we are working on some leads, the sources said.