Three militants captured during attacks in Mumbai have reportedly confessed to being members of the Pakistan-based Lashkar-e-Taiba group.
Indian Prime Minister Manmohan Singh said the attackers came from "outside the country".
Mr Singh said the attacks were the work of militant groups based in the territory of India's neighbours, usually an allusion to Pakistan, raising prospects of renewed tension between the nuclear-armed rivals.
Lashkar-e-Taiba has denied any role in the attacks.
The Hindu newspapers reports one of the attackjers was a resident of Faridkot in Pakistan's Punjab province.
"Based on the interrogation of the suspects, the investigators believe that one or more groups of Lashkar operatives left Karachi in a merchant ship early on Wednesday," the newspaper said.
It said the group came ashore at Mumbai on a small boat and then split up into small teams to attack different targets.
The Indian Express newspaper said the group left the Pakistani port city of Karachi by sea and transferred to two small boats or rubber dinghies off Mumbai. They were seen by several residents coming ashore but allayed suspicion by saying they were students, it said.
Ships stopped by Navy
The Indian navy has apprehended two Pakistani merchant ships suspected of involvement in the attacks. The ships were stopped in waters just north of Mumbai.
The Times of India said the attackers were aged between 18 and 25. Each was given "an AK-47 assault rifle with two magazines each, one pistol and eight to 10 grenades suspected to have manufactured at a Pakistan ordnance factory," it said.
Wrote strategic affairs analyst K. Subrahmanyam in the same newspaper: "The equipment, training and sophistication of their planning would tend to indicate a Pakistani link,"
Lashkar-e-Taiba, along with another group, Jaish-e-Mohammed, made its name fighting Indian rule in disputed Kashmir, where state elections are under way.
Both groups were closely linked in the past to the Pakistani military's powerful Inter-Services Intelligence agency, the ISI.
The groups were blamed for an attack on India's parliament in 2001 which brought the two countries close to a fourth war since independence from Britain 60 years ago.
"The possibility of rogue elements in ISI and jihadi elements in Pakistan conspiring to create tensions between New Delhi and Islamabad cannot be ruled out," Mr Subrahmanyam wrote.
Mr Singh did not specifically name Pakistan, which has condemned the attacks and promised full co-operation.
"We will take up strongly with our neighbours that the use of their territory for launching attacks on us will not be tolerated, and that there would be a cost if suitable measures are not taken by them," Mr Singh said.
Pakistan has denied any involvement in the attacks.
Its ambassador to the United States, Hussein Haqqani, says now is not the time to point any fingers of suspicion, and it is unfair to blame anyone without a proper investigation.