MUMBAI (Reuters) - Prime Minister Manmohan Singh vowed on Thursday to bring to justice those behind triple bomb attacks on Mumbai, and police questioned members of home-grown Islamist militant group Indian Mujahideen.
No group has claimed responsibility for Wednesday's attacks, the most deadly since Pakistan-based militants struck India's financial hub in 2008, killing 166 people and raising tensions with nuclear rival Pakistan.
Authorities have yet to say publicly who they believe was responsible for the three near-simultanous blasts during the evening rush hour, which killed 18 people and injured 133 others.
The blasts have heaped pressure on Singh as he struggles to overcome a series of graft scandals that have boosted a resurgent opposition and led to policy paralysis in Asia's third largest economy.
"The terrorists had the advantage of surprise," Singh said in rare public comments outside 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," he said.
As police sifted forensic evidence and security camera footage, Home (interior) Minister Palaniappan Chidambaram said it was too early to point the finger at a particular group.
The "coordinated terror attacks" could be retaliation for police action that led to a number of arrests and disrupted a plots, he said, adding that the lack of prior warning did not represent a failure by the intelligence agencies.
The home ministry said in a statement police were interrogating some Indian Mujahideen members who were arrested days before the attack, but that it had no specific leads on who might be responsible.
The Indian Mujahideen is a home-grown militant group known for its city-to-city bombing campaigns using small explosive devices planted in restaurants, at bus stops and on busy streets.
The group has been accused of ties to Pakistani militant groups involved in attacks in Indian Kashmir as well as elsewhere in the country.
"It's very likely coordinated by Indian Mujahideen looking at the severity and scale of the attacks -- in the past they've used tiffin carrier bombs and IEDs," said Rohan Gunaratna, a Singapore-based al Qaeda expert. Tiffin carriers are steel containers used to carry lunch in India.
The bombings were the most deadly attacks on Mumbai since the 2008 assaults killed 166 people and raised tensions with nuclear rival Pakistan.
After a two-year chill, India and Pakistan have been trying to normalise ties and later in July their foreign ministers are due to hold talks.
Pakistani leaders were swift in condemning the bombings, as was U.S. President Barack Obama. Top U.S. Secretary of State Hillary Clinton is also due in India for talks next week.
Any suggestion Pakistan-based groups were involved in the attack would complicate Islamabad's already fraught relationship with New Delhi and further unravel Pakistan's ties with the United States.
"We live in the most troubled neighbourhood in the world. Pakistan and Afghanistan are the epicentre of terrorism," said Chidambaram, adding that Pakistan had still not given India support in pursuing those behind the 2008 attacks in Mumbai.
The Hindu nationalist Bharatiya Janata Party (BJP) accused the government of being lax.
"These repeated attacks on Bombay (Mumbai) should be viewed as a policy failure. It is not an intelligence failure," said top BJP leader L.K. Advani, a former deputy prime minister.
Mumbai, a coastal city of 20 million people that is home to India's main stock exchanges, has a long history of deadly bombings and Wednesday's attacks did not rattle financial markets.
The bombings were centered mainly on south Mumbai's bustling jewellery market districts, crowded with diamond and precious metals traders and artisans.
"These IEDs were not crude devices, but it seems that they were made with some sophistication. Those who made them had prior training," Home Secretary Raj Kumar Singh, the ministry's top civil servant, told reporters. He said they were detonated by some sort of timer device.
Police were investigating whether electric wires found attached to a body had anything to do with the bombs, he said. U.K. Bansal, India's top internal security official, did not rule out the possibility of a suicide bomber but said there was no firm evidence yet.
Writing by Paul de Bendern; Editing by Jon Boyle; additional reporting by James Pomfret, Annie Banerji and C.J. Kuncheria in NEW DELHI, Rosemary Arackaparambil, Rajendra Jadhav, Tony Munroe and Jui Chakravorty in MUMBAI