U.S. counterterrorism officials are stunned at the level of coordination in the Mumbai attacks and fear that Indian Muslim militants may be changing tactics to focus their wrath on Westerners.
The U.S. officials also told ABC News they fear that the group which paralyzed the city of Mumbai with its bloody offensive may have had funding and logistical support from Al Qaeda.
The gunmen who carried out the assaults identified themselves as Deccan Mujahideen, a previously unknown group. Mujahideen means holy warrior, and Deccan refers to a plateau in southern India.
Whoever they are, intelligence agencies around the world are scrambling to identify the source of this latest threat.
The onslaught is remarkable for its coordination and sophistication.
The American officials point out that the militants attacked 10 sites almost simultaneously, at least some of the assailants arrived at their targets by boat, and at some of the sites they set up command centers to coordinate attacks and may have allowed them to communicate from site to site.
The attacks also appear to mimick al-Qaeda tactics -- simultaneous attacks on high profile targets in the heart of a city's financial district.
The Mumbai sites attacked are popular with Westerners, and survivors said the gunmen were searching for anyone with an American or British passport.
U.S. officials believe the attackers were Kashmiri jihadists who have been battling India for years over the disputed region of Kashmir, along the border with Pakistan. The Kashmiri militants are based in Pakistan and have a history of receiving support from Pakistan's intelligence agency.
What is deeply disturbing to U.S. law enforcement officials, however, is that these militants may now be targeting Westerners, particularly Americans, to draw attention to their cause, despite not having a specific grievance against the United States.
An attack on Westerners would have a bigger impact and draw worldwide coverage of their complaints, the officials theorize.
Because of the sophistication of the attacks, sources told ABC News that U.S. officials are looking into the possibility that Kashmiri militants may be strengthening ties with al Qaeda, and may have carried out the attack with funding and logistical support from al Qaeda.
They also estimate the widespread carnage could have been carried out for the relatively small price tag of $100,000 to $200,000, the cost of the machine guns, grenades, ammunition and boats used.
The FBI is preparing a small team of investigators and forensic scientists to go to Mumbai.
India has suggested the attackers may have had support in Pakistan, which has fought two was with India.
Indian Prime Minister Manmohan Singh blamed "external forces" but stopped short of blaming Pakistan. And the Indian navy said it is investigating a ship that recently arrived from Karachi, Pakistan and is suspected of having brought the attackers to Mumbai.
Pakistan's foreign minister, Shah Mehmood Qureshi, in New Delhi for peace talks, told the Dawn television station that nobody should be blamed until investigations were complete.