The attacks have increased tension between India and Pakistan, both nuclear states which have fought two wars with each other. It was "evident" that a "group based outside the country" carried out the attacks, Indian Prime Minister Manmohan Singh said Thursday in an address to the nation.
Singh didn't elaborate or name Pakistan, but he said India would "take up strongly with our neighbors that the use of their territory for launching attacks on us will not be tolerated and that there will be a cost if suitable measures are not taken by them."
In an effort to defuse tension, Pakistani President Asif Ali Zardari called Singh to say he was "appalled and shocked" by the slaughter and would cooperate with India to track down the masterminds of the attacks.
In addition, the director of Pakistan's powerful intelligence agency will go to India to help investigate, Pakistan announced.
Britain's security services are working "intensively" with Indian authorities to determine whether several of the attackers were British citizens with ties to India or the disputed Himalayan territory of Kashmir. "It's too early to say whether or not any of them are British," said Britain's Foreign Secretary David Milliband.
Indian government sources told ABC News they believe the explosives arrived by boat. Investigators found a rubber boat with explosives in it just 100 feet from the Taj hotel.
The Indian Navy said its forces boarded a cargo vessel suspected of ties to the attacks. Navy spokesman Capt. Manohar Nambiar said Thursday that the ship, the MV Alpha, had recently come to Mumbai from Karachi, Pakistan.
The gunmen came prepared for a siege, officials said, even hauling in large bags of almonds to feed themselves during a long gunbattle.
"They have AK-47s and grenades. They have bags full of grenades and have come fully prepared," said Maj. Gen. R.K. Hooda.
Ratan Tata, who runs the company that owns the Taj Mahal, said they appeared to have scouted their targets in advance.
"They seem to know their way around the back office, the kitchen. There has been a considerable amount of detailed planning," he told a news conference.
Survivors told of chaos inside the hotels with the attacks erupting as many sat down to meals in the hotels' dining rooms. They described dead bodies in hallways, by the pool, of hiding behind tables covered with table cloths, of marble and sleek wooden floors streaked with blood.
Some were trapped by terrorists while others were trapped by the fires that burned out of control. Hotel customers were seen in their windows signaling desperately for rescue while others flicked their lights on and off in a distress signal.
One group described slinking downstairs, their shoes in their hands to muffle the noise and avoid alerting the terrorists.
Bobbie Garvey, a spokewoman for the Synchronicity Foundation, said 25 members of their meditation group were in the Trident-Oberoi. Two were killed and four wounded. The others were barricaded in their rooms with mattresses and bureaus pushed up against their hotel room doors.
She said they spent three days in terror. "They didn't know if that door would open at any time and it would be someone to save them or someone to take them out," Garvey said.
The hostage situation played out after a series of coordinated terrorist attacks that paralysed much of southern Mumbai.