"It would be premature in view of the unfolding tragedy in Mumbai and the corresponding investigation to reach any hard-and-fast conclusions on who may be responsible for the attacks, but some of what we're seeing is reminiscent of past terrorist operations undertaken by groups such as Lashkar-e-Taiba and Jaish-e-Mohammed," a U.S. counterterrorism official said on condition of anonymity. The two groups mentioned by the official are Pakistani militants linked to al-Qaida who have fought Indian troops in Kashmir.

India's cities are no strangers to indiscriminate terror attacks. Such attacks have occurred regularly, and with steadily increasing frequency, in recent years.

"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.