Intelligence agencies received no warnings before the three bomb blasts that killed 18 people in Mumbai, the biggest attack since Pakistani-based militants rampaged through the financial hub in 2008, home minister P Chidambaram said on Thursday.
"There was no intelligence regarding a militant attack in Mumbai," Chidambaram told a news conference in Mumbai.
"(Maybe those) who perpetrated the attacks worked in a very clandestine manner, maybe a very small group that has not communicated with each other."
He said it was too early to point the figure at any particular group, but said the coordinated attacks could be in retaliation to a number of plots stopped by police or the arrests of militants, including members of the Indian Mujahideen, a home-grown group said to have support from militants in Pakistan.
Chidambaram said 18 people had died in the triple attacks, lowering an earlier finger of 21. He said 23 out of the 131 injured and admitted to hospitals were in a critical state.
The bombings were the biggest militant attack on Mumbai since the 2008 assaults killed 166 people, raised tensions with neighbour and nuclear rival Pakistan, and left a city on edge.
There was no immediate claim of responsibility for the blasts.
Newspaper headlines voiced a mix of resignation and outrage over the latest attacks on a city of more than 10 million that is home to the main stock exchange.
"Attacked. Again," said the Hindustan Times.
"We're All Sitting Ducks," said the Economic Times.
The blasts came as beleaguered Prime Minister Manmohan Singh struggles to get past a series of corruption scandals and a resurgent opposition that has led to policy paralysis in Asia's third largest economy.
A cabinet reshuffle this week was criticised as too little, too late.
Stock and commodity exchanges opened as normal on Thursday and schools as well, although heavy monsoon rains led to delays and cancellations of train services during the morning rush hour.
Mumbai has a long history of deadly bombings and Wednesday's attacks were unlikely to rattle financial markets.
The bombings, centered mainly on south Mumbai's jewellery market area, were described by the home minister as "coordinated terrorist attacks."
"This is another attack on the heart of India, heart of Mumbai. We will fully meet the challenge, we are much better prepared than 26/11," Prithviraj Chavan, the state's chief minister, told NDTV on Wednesday evening, referring to the 2008 attacks.
The blasts occurred at about 6.45pm on Wednesday within minutes of each other. At least one car and a motorbike were used in the coordinated attacks, officials said.
The biggest blast was in the Opera House area, a hub for diamond traders. Pakistani-based militants carried out the bloody rampage in 2008 near the same popular area.
Another blast, also in south Mumbai, was at the Zaveri Bazaar, India's largest bullion market which was hit twice in the past. The third blast was at Dadar, in a crowded street housing Muslim and Hindu shops in the centre of the coastal city.
There was no immediate indication any Pakistani group was involved. But any suggestion of attributing blame to Islamabad would complicate a fraught relationship with India -- with whom it has a long-running dispute over Kashmir -- and further unravel ties with the United States.
The US has withheld some military aid to Pakistan to pressure it to buckle down in the war on terror.
US President Barack Obama condemned the attacks and offered support to bring the perpetrators to justice.
Pakistan's President Asif Ali Zardari and Prime Minister Syed Yousaf Raza Gilani swiftly condemned Wednesday's blasts in a statement.