Westerners in India's financial center were targeted in the- a signature of past al Qaeda actions including the Sept. 11 attacks. But the Indian attack was carried out by gunmen and not the suicide bombers frequently employed by al Qaeda and its affiliates.
More than 100 people have been killed with at least 300 people wounded.
The group that claimed responsibility - Deccan Mujahideen - was unknown to security officials, a British security official told The Associated Press on condition of anonymity because of the sensitivity surrounding the work. He said terror threats in India had been increasing but the scale of the attack on Wednesday was a surprise and there were no indications attacks would target Westerners.
"We have been actively monitoring plots in Britain and abroad and there was nothing to indicate something like this was about to happen," the official told the AP.
CBS News correspondent Bob Orr reported that U.S. officials had not heard of Deccan Mujahideen before Wednesday.
Britain is the former colonial power in India and Pakistan and closely monitors terrorist suspects in those countries. The majority of the nearly 2 million British Muslims are of Indian, Pakistani and Bangladeshi origin. More than 2,000 terror suspects are being monitored in the UK alone, with dozens more being watched in other countries, Britain's security services have said.
Another British security official told the AP on condition of anonymity that, though it is too early to know for sure, the attack doesn't look to have been directed by al Qaeda's core leadership. But he said the fact Westerners had been singled out suggested it was inspired by Islamic extremist ideology.
Western security officials believe attacks organized, directed and funded specifically by al Qaeda's core leadership along the Afghan/Pakistan border are not frequent. More common are incidents in which terrorists have either some limited contact with al Qaeda leaders, or are inspired to carry out attacks by the ideology of Islamic extremism.
A U.S. counterterrorism official warned against leaping to conclusions but said the Mumbai attacks bore some hallmarks of operations by Pakistani groups that have fought Indian troops in the divided Kashmir region.
"Some of what we're seeing is reminiscent of past terrorist operations undertaken by groups such as Lashkar-e-Taiba and Jaish-e-Mohammed," the official said on condition of anonymity because of the sensitivity of the matter.
Indian Prime Minister Manmohan Singhand the Indian navy said its forces were boarding 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.
Pakistan's Port and Shipping Minister Nabil Gabol said Indian authorities had not asked him for information about what he called a "false allegation."
"They should not drag Pakistan into this just to overcome their own political problems," he told The Associated Press.
Pakistani officials in Britain said they were unaware of the plot. In September, a massive suicide truck bomb devastated the Marriott Hotel in the capital, Islamabad, killing at least 54 people, including three Americans and the Czech ambassador.
"This type of terrorism is spreading, through Pakistan and now India, but we were all surprised by such a large-scale attack like this," said Wajid Hassan, Pakistan's High Commissioner in London. "This is no coincidence that this type of attack happened so soon after the bombing of the Marriott Hotel. People from all countries are being paid to fight this al Qaeda war. This is a war that goes beyond any nationality."
Few terrorism experts have heard of the Deccan Mujahideen.
"Initially we saw violence in India imported from outside - with allegations of Pakistani government support - but now we are seeing new, homegrown groups," said Nigel Inkster, director of Transnational Threats at the International Institute for Strategic Studies in London.
"There is a possible link to al Qaeda," he said. "Logically it would be easier for al Qaeda to get things done in India than in the U.S. and Europe. Everyone's been expecting some type of pre-U.S. election or post-U.S. election spectacular, and there is some speculation that this is it."