The coordinated series of attacks, which began Wednesday night, targeted 10 sites across Mumbai, including an iconic hotel and a landmark train station. More than 150 people were killed in the rampage, including at least 16 foreigners - four of them Americans.
Local media speculation quickly settled on Lashkar-e-Tayyeba, a Pakistan rebel group that has fought troops in Indian-controlled Kashmir, but newspapers and TV channels have offered little evidence for the suspicion.
Indian federal home minister Jaiprakash Jaiswal said a captured gunmen had been identified as a Pakistani while R. R. Patil, a top official in Maharashtra state, said, "It is very clear that the terrorists are from Pakistan. We have enough evidence that they are from Pakistan."
Earlier, Prime Minister Manmohan Singh blamed "external forces" for the violence - a phrase sometimes used to refer to Pakistani militants, whom Indian authorities often blame for attacks.
India's Foreign Minister Pranab Mukherjee, said "some elements in Pakistan are responsible for Mumbai terror attacks," but he added that "proof cannot be disclosed at this time.
Mukherjee's carefully phrased comments appeared to indicate he was accusing Pakistan-based groups of staging the attack, and not Pakistan itself.
None of the officials provided further details.
A previously unknown group calling itself the Deccan Mujahideen has claimed responsibility for the attacks.
On Thursday a man identifying himself as spokesman for the Lashkar rebel group denied any role in the attacks in an interview to local Hindi news channel, in Indian Kashmir's main city Srinagar over telephone.
Lashkar has long been seen as a creation of the Pakistani intelligence service to help wage its clandestine war against India in disputed Kashmir.
Any clear proof of Pakistani involvement would seriously jeopardize fragile peace talks between the nuclear-armed rivals.
Pakistan has vigorously denied all accusations that it was tied to the attacks.
"I will say in very categoric terms that Pakistan is not involved in these gory incidents," said Pakistan's Defense Minister Ahmed Mukhtar.
Hindu-majority India and Muslim Pakistan were born in the bloody partition of the subcontinent at independence from Britain in 1947. They have fought three wars, two of them over the disputed Himalayan region of Kashmir and held tit-for-tat nuclear weapons tests before peace talks got under way in 2004.
India has long accused Islamabad of allowing militant Muslim groups, particularly those fighting in Kashmir, to train and take shelter in Pakistan.
New Delhi's past complaints about Pakistan have centered on its Inter Services Intelligence agency.
In an apparent goodwill attempt, Pakistan said Friday that it would send its spy chief, Lt. Gen. Ahmed Shujaa Pasha, to India to help probe the attacks. No date has been set for the visit.
Analysts debated the level of Pakistan's possible involvement.
The terror group "itself is probably drawing from, in large numbers, Indian operatives, but it probably enjoys a fairly healthy support of Pakistan," Christine Fair, a South Asia specialist at the RAND Corp., said Friday. "The big picture is that there's probably going to be more of this, not less of this, to come."
The British government, meanwhile, was investigating whether some of the attackers could be British citizens with links to Pakistan or Kashmir, a British security official told The Associated Press on condition of anonymity because of the sensitivity of his work.
A U.S. investigative team was heading to Mumbai, a State Department official said Thursday evening, speaking on condition of anonymity because the U.S. and Indian governments were still working out final details.
India has been shaken repeatedly by terror attacks blamed on Muslim militants in recent years, but most were bombings striking crowded public places: markets, street corners, parks. Mumbai - one of the most populated cities in the world with some 18 million people - was hit by a series of bombings in July 2006 that killed 187 people.
But these latest attacks were significantly more sophisticated - and more brazen.
They began at about 9:20 p.m. with shooters spraying gunfire across the Chhatrapati Shivaji railroad station, one of the world's busiest terminals. For the next two hours, there was an attack roughly every 15 minutes - the Jewish center, a tourist restaurant, one hotel, then another, and two attacks on hospitals.
The gunmen apparently came to Mumbai by boat, and Indian forces expanded their investigation to the sea in the hours and days after the attack.
Authorities stopped a cargo ship off the western coast of Gujarat that had sailed from Saudi Arabia and handed it over to police for investigation, said Navy Capt. Manohar Nambiar.
They also stopped a cargo ship that had come to Mumbai from Karachi, Pakistan, but released it when nothing suspicious was found on board.