Mumbai: Investigators into the 13 July bomb blasts in Mumbai have narrowed their focus onto underworld elements and homegrown terrorist outfit Indian Mujahideen (IM). The three explosions during the evening rush hour killed 18 and left 131 hurt, according to PTI.

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Officials of the National Investigation Agency (NIA), the Maharashtra Anti-Terrorisism Squad (ATS), the Mumbai police crime branch and the Intelligence Bureau (IB) have been questioning people with connections to the underworld. The owner of the scooter that was used to plant explosives at Zaveri Bazaar was also interrogated.

Terror probe: Forensic experts inspect the blast site near Opera House in Mumbai on Friday morning. Photo PTI

U.K. Bansal, secretary, internal security, said investigators are not ruling out the possibility of a previously unknown group behind the attacks.

“We do not know who is behind it. If we do not get any information, then chances are high that it could be a new group," he said.

A high-ranking NIA official said, on condition of anonymity, “We still do not have any signature of the blast."

The forensic experts who had flown in from elsewhere have left Mumbai.

“We have examined the exhibits and told the investigators that it could be the handiwork of Indian Mujahideen. But we are not very sure," said the official cited above.

Teams were sent to Delhi, Uttar Pradesh and Madhya Pradesh to look for possible clues.

Asked whether there were any cross-border links to the blasts, the home secretary said: “We have an email which originated elsewhere. It is being followed up".

Singh said the CCTV footage of the three blast sites was being scrutinized by the investigators.

“We have all the people and the faces. They have to be recognized by local people to see whether they are locals or outsiders. That process is on," he said.

The home secretary said people not recognized by the locals have been put through a database and were being verified.

“Suspicious people are being verified. Various people with previous known linkages are being questioned based on our previous databases. This investigation is on," he said.

The Congress party on Friday condemned the blasts in Mumbai.

“We strongly condemn this cowardly act. We are hopeful that the agencies of the state and Central government will investigate this in detail and take strict action against those who are responsible for this," Congress spokesperson Shakeel Ahmed told reporters. He added that the matter should not be politicized.

In Delhi, the opposition Bharatiya Janata Party spokesperson said: “The BJP will support the government in the event it takes strong action along with all the instruments necessary to secure national security under attack from terrorists and their patrons from across the border. However, the most important question is, does the present UPA (United Progressive Alliance) government possess the necessary political will?"

Maharashtra chief minister Prithviraj Chavan said the state was considering equipping emergency services with satellite phones or developing a secure communication network as there had been “complete collapse" of mobile communication on 13 July evening for the first 15 minutes after the terrorist attack.

Everything that has been recommended (in the Ram Pradhan committee report on the 26/11 attack in Mumbai) has not been done, such as the purchase of CCTV cameras and the police modernization programme has not moved as rapidly as it should have, Chauhan said.

Home secretary Singh said traces of TNT (trinitrotoluene) had been traced at the Zaveri Bazaar and Opera House sites, where the impact of the blasts was high. Ammonium nitrate and shrapnel were also used, he said.

Ruhi Tewari, Anuja from Delhi and PTI contributed to the story.