Counterterrorism officials and experts said the scale, sophistication and targets involved in the Mumbai attacks were markedly different from previous terrorist plots in India and suggested the gunmen had received training from outside the country. But they cautioned it was too soon to tell who may have masterminded the operation, despite an assertion from a previously unknown Islamist radical group.
Officials in India, Europe and the United States said likely culprits included Islamist networks based in Pakistan that have received support in the past from Pakistan's intelligence agencies.
Analysts said this week's attacks surpassed previous plots carried out by domestic groups in terms of complexity, the number of people involved and their success in achieving their primary goal: namely, to spread fear.
"This is a new, horrific milestone in the global jihad," said Bruce Riedel, a former South Asia analyst for the CIA and National Security Council and author of the book "The Search for Al Qaeda." "No indigenous Indian group has this level of capability. The goal is to damage the symbol of India's economic renaissance, undermine investor confidence and provoke an India-Pakistani crisis."
Several analysts and officials said the attacks bore the hallmarks of Lashkar-i-Taiba and Jaish-i-Muhammad, two networks of Muslim extremists from Pakistan that have targeted India before. Jaish-i-Muhammad was blamed for an attack on the Indian Parliament in 2001.
'Army of the Pious'
Both groups have carried out a long campaign of violence in the disputed territory of Kashmir, which India and Pakistan have fought over for six decades. The roots of the long-running conflict are religious: A majority of India's population is Hindu, while most Pakistanis are Muslim.
A U.S. counterterrorism official, speaking on the condition of anonymity, said Lashkar-i-Taiba, which means "Army of the Pious," and Jaish-i-Muhammad, or "Soldiers of Muhammad," are "the thing people are starting to look at. But I can't caution enough to treat it as a theory, a working assumption. It's still too early for hard and fast" conclusions.
"What the Indians have in their favor," the official added, "is that they've got some of these guys. It seems logical that they can expect to work their way back reasonably quickly." Indian officials said several gunmen were captured.
In its Friday editions, the newspaper the Hindu reported that at least three of the suspects held by police were members of Lashkar-i-Taiba and that the assailants had arrived in Mumbai on a ship from Karachi, Pakistan.
Earlier, Pakistan's government condemned the attacks and warned India against jumping to conclusions about who was responsible. Lashkar-i-Taiba issued a statement denying involvement.
India has been plagued by a wave of terrorist attacks in recent years, many sparked by friction between Hindu nationalists and minority Muslim groups. The shootings in Mumbai were far from the worst to strike India's financial capital; bombings in 1993 and 2006 each killed more than 180 people.
A group calling itself the Deccan Mujaheddin asserted responsibility for the attacks in e-mails sent to Indian media organizations Wednesday. Officials said they had never heard of the group.
Television footage showed the assailants carrying automatic rifles and backpacks filled with ammunition and grenades. Analysts said the fact that the gunmen quickly fanned across the city and were able to hold off Indian security forces over three days suggested that they had received training at organized camps.
"What is striking about this is a fair amount of planning had to go into this type of attack," said Roger W. Cressey, a former White House counterterrorism official in the Clinton and Bush administrations. "This is not a seat-of-the-pants operation. This group had to receive some training or support from professionals in the terrorism business."
Some experts said the operation bore resemblances to plots orchestrated by al-Qaeda, in that it involved multiple, simultaneous attacks targeting foreigners. In this case, according to witnesses, the gunmen sought out Americans and Britons, and also took hostages at the local headquarters of an Orthodox Jewish group.
Others said they were dubious of a connection to Osama bin Laden's organization. They said al-Qaeda has relied on suicide bombers, not gunmen, and is not known to have cells in India.
David Miliband, Britain's foreign secretary, told reporters that it was "premature to talk about links to al-Qaeda" and that it was still unclear who the intended targets were. "This is only the latest in a series of attacks in India over the last year or two," he said, adding, "Terrorism is not just a war against the West."
Long list of suspects
Peter Neumann, a terrorism analyst at King's College in London, noted that dozens of gunmen were involved. "This doesn't mean it's al-Qaeda, or they take orders from bin Laden, but I'm pretty sure it's not some leaderless, grass-roots thing."
On Wednesday, al-Qaeda's propaganda arm released a video on the Internet featuring an interview with Ayman al-Zawahiri, the network's deputy leader. He made no mention of the attacks in Mumbai; it was unclear when the video was produced.
Other experts warned that there is a long list of suspects who could have played a role. For instance, Indian officials have blamed the 1993 bombings in Mumbai, which killed 257 people, on Dawood Ibrahim, an organized crime figure who remains on the run.
"Anything could be in the cards," said Magnus Ranstorp, a terrorism analyst at the Swedish National Defense College. "With most terrorist attacks, it's relatively clear-cut who is involved. In this case, it could be all sorts of constellations that are at work."
DeYoung reported from Washington. Special correspondent Karla Adam in London contributed to this report.