Analysts are divided over whether the hand of al-Qaeda can be detected. The only claim of responsibility comes from a group that may not even exist: an e-mail message claiming responsibility and sent to Indian media on Wednesday night said the attackers were from a group called Deccan Mujahideen.
Deccan is a neighborhood of the Indian city of Hyderabad. The word also describes the central and southern region of India, which is dominated by the Deccan Plateau. Mujahideen is the commonly used Arabic word for holy warriors.
But Sajjan Gohel, a security expert in London, called it a "front name" and said the group was "nonexistent."
Alex Neill, head of the Royal United Services Institute's Asia security programme, believes the attacks were probably carried out by local jihadists linked to the radical Students Islamic Movement of India (Simi), a banned Islamic fundamentalist organisation which advocates the "liberation of India" by converting it to an Islamic state.
One possible mastermind and Simi member is Abdul Subhan Qureshi, a 36-year-old computer engineer suspected of being behind multiple bombings in Delhi, Jaipur, Bangalore and Ahmedabad earlier this year.
Qureshi, also known as Tauqeer, is from Mumbai and his expertise with internet security could have played a vital part in pulling off such an ambitious plot, said Mr Neill.
"He is an IT whizz-kid so it is quite possible he is the person investigators will be concentrating on. This is a great embarrassment to the Indian security services because it has been pulled off right under their noses."
Simi has declared jihad on India, the aim of which is to establish Dar-ul-Islam (land of Islam) by forcefully converting everyone to Islam.
Mr Neill said Deccan Muhajideen would be a militant offshoot of Simi which has carried out attacks across India. He added: "The perpetrators have obviously been highly trained and would have been sent to al-Qaeda training camps to prepare. I would be astonished if any of them are from Britain – they were probably recruited from the Mumbai region."
He reckons up to 100 terrorists would have been involved in the planning and execution of the attack and said it was surprising they had managed to keep it a secret.
Other analysts say that while it is not clear whether the Deccan Mujahideen claim is genuine, the attacks may have been carried out by a group called the Indian Mujahideen – also an offshoot of Simi and blamed by police for almost every major bomb attack in India, including explosions on commuter trains in Mumbai two years ago that killed 187 people.
Police said the Indian Mujahideen may also include former members of Bangladeshi militant group, Harkat-ul-Jihad al Islami. In an e-mail in September, the group denounced Mumbai's police anti-terrorist squad (ATS), accusing them of harassing Muslims.
"If this is the degree your arrogance has reached, and if you think that by these stunts you can scare us, then let the Indian Mujahideen warn all the people of Mumbai that whatever deadly attacks Mumbaikars will face in future, their responsibility would lie with the Mumbai ATS and their guardians," it said.
The Mumbai attacks appear to have been carefully coordinated, well-planned and involved a large number of attackers. A high level of sophistication has also been a hallmark of previous attacks by the Indian Mujahideen.
The Mumbai attacks also focused clearly on tourist targets, including two luxury hotels and a famous cafe.
In May, the Indian Mujahideen made a specific threat to attack tourist sites in India unless the government stopped supporting the United States in the international arena.
The threat was made in an e-mail claiming responsibility for bomb attacks that killed 63 people in the tourist city of Jaipur. The e-mail declared "open war against India" and included the serial number of a bicycle used in one bombing.
Indian Prime Minister Manmohan Singh has blamed a group with "external linkages" for the attacks. He said: "It is evident that the group which carried out these attacks, based outside the country, had come with single-minded determination to create havoc in the commercial capital of the country."
He could have been referring to either Pakistan or Bangladesh, which has also been accused by India of harbouring militant groups. Some security specialists believe there is likely to have been a degree of inspiration from, or link with, external groups allied to al-Qaeda, such as the Pakistan-based Lashkar-e-Taiba, which wants to see India expelled from Kashmir.
Eyewitnesses have reported hostage-takers speaking with a Kashmiri accent. However, Lashkar-e-Taiba yesterday denied any role in the Mumbai attacks.
Henry Wilkinson, a senior analyst with Janusian Security Risk Management, a London-based consultancy, said the tactics are different from the more common, post-9/11 attacks seen in Iraq and Afghanistan, but bear similar hallmarks. He said: "It's very interesting that they didn't go in using car bombs; it was more of a direct armed assault on a city. It's very reminiscent of the attacks in Saudi Arabia in 2003, when the gunmen were going around trying to find Westerners and kill them."
Defiant and resilient city with people, not symbols, at its heart
JUST last week the former United States secretary of state Henry Kissinger sat with top executives from Goldman Sachs and India's Tata group to chat about American politics in one of the Taj Hotel's many opulent meeting rooms.
Last night, that wing of the building was engulfed in flames.
Supriya Nair, a Mumbai journalist, commenting on the fire, characterised the city's celebrated resilience.
"The Taj itself has been rebuilt a couple of times after accidents and the like over the last century." he said. "This is not a city that has ever cared much for symbols. We build them all the time, they wash away, we build again. It's the people who are the bricks of the city."
How much the attacks will affect India's growth prospects in the short term is still in question.
Sharon Bamford, the chief executive of the UK India Business Council (UKIBC), said the attacks were a "wake-up call" for the Indian government on security issues, but added they will not jeopardise the multi-billion-pound trade ties between the UK and India.
She cited the case of one British businesswoman, a member of UKIBC, who was caught up in the events at the Taj hotel and had to shelter under a restaurant table for more than six hours.
"Knowing the character of our intrepid business leaders and entrepreneurs in India, I know that she will bounce back and her business will continue," Ms Bamford said.
Her confidence was echoed by Jan Masiel, a member of the European Parliament, who is visiting Mumbai with a trade delegation of eight. "I don't think this affects India's image as a good place to do business in … neither do we consider India to be an insecure or unsafe country to be in," said the Polish national, who was waiting to go back into the Taj Mahal hotel.
The attacks are "a challenge to the economic resurgence in India", said Habil Khorakiwala, the chairman of Wockhardt, an Indian pharmaceuticals company.
"The targets identified clearly demonstrate that the intention is to create panic and shatter the confidence in the minds of investors," he said.
"This war has to be fought together by all, to protect the safety of the Indian people, for economic resurgence and the growth of the Indian nation."
Britain is India's fifth-largest trading partner, and India is Britain's 18th-largest export market, its second-largest export market in the developing world after China.
MUMBAI ATTACKS: MORE COVERAGE
• He looked like any student … seconds later he became the young face of terrorism
• I'm safe – wealthy Briton's call home hours before he was shot dead
• India: A prime target for the purveyors of terror
• Kashmir a source of anger for militant Muslims
• Scots may be caught in violence
• Terrorism jeopardises cricket's future