Maharashtra Anti-Terrorism Squad, ATS has taken over the investigations into the Wednesday's serial bombings in Mumbai that left 18 people dead and 131 injured. This was disclosed by ATS Chief Rakesh Maria in Mumbai yesterday. He said, 12 joint teams of ATS and Crime Branch have been formed and Central agencies are assisting them. Investigators are sifting through the CCTVs installed near the blast sites, particularly the footages at the Opera House which may give some vital clues. An ATS official said last night that while going through the footage, investigators have seen suspicious behaviour of some people. But until they question them or record their statement, nothing can be said about their involvement. Union Home Secretary RK Singh does not rule out the involvement of a suicide bomber. He said, the discovery of a wired electrical circuit on the body of one of the dead persons raised questions whether a human bomb was used to carry out the attack. ATS Chief Rakesh Maria said, it is too early to say whether a human bomb was used. There is no specific lead yet and authorities does not want to speculate on who could be involved in the attack. Union Home Minister P Chidambaram said it is too early to point a finger at any one group and all groups hostile to India are on the radar. He said, ammonium nitrate was used in the IED which was triggered by timer devices to carry out the blasts. Secretary, Internal Security in the Home Ministry U K Bansal said, a few Indian Mujahideen militants arrested some time ago are being interrogated. The Prime Minister reviewed security situation in Mumbai at a meeting convened at the city airport. Maharashtra Governor K Sankaranarayanan, Chief Minister Prithviraj Chavan, his deputy Ajit Pawar and Home Minister R R Patil were among those present in the meeting.