Indian police say the gunmen who terrorized Mumbai and killed at least 172 people travelled to the city by boat from Pakistan, and may have had no intention of escaping with their lives.

"It appears that it was a suicide attack," Mumbai police commissioner Hasan Ghafoor said Tuesday.

The news broke after an American government official said the U.S. warned India about a possible terrorist strike on the city before last week's siege.

The senior official, who spoke on the condition of anonymity, said the U.S. passed along information that terrorists appeared to be plotting a waterborne attack, according to The Associated Press. There was no information about when or how the U.S. learned of the plot.

The government of India is calling on Pakistan to hand over 20 suspected terrorists, including a man that is on New Delhi's "most wanted" list.

On Monday night, India's foreign ministry sent Islamabad a list of people it believes may help in the investigation into the attacks on Mumbai.

"These are people who belong to pan-militant organizations," ABC News producer Habibullah Khan told Canada AM by phone from Islamabad on Tuesday.

"Of course, Pakistan has banned all militant organizations. All the 20 people are supposed to be members of those banned militant organizations."

One of the wanted men is Dawood Ibrahim, the alleged mastermind of 1993 Mumbai bombings and India's most-wanted gangster.

Pakistan has denied its government or agencies were involved in the attacks. Despite the denials, the international community is concerned about rising tensions between the two nuclear powers, which have gone to war in the past over the disputed Kashmir region.

"There have not been any indications of troop movements at the borders ... so that is a good sign," Khan said, noting that India and Pakistan have not overtly changed their military postures since the Mumbai attacks.

Earlier on Tuesday, AP also reported that India's foreign intelligence had received information in September that Pakistan-based terrorists were plotting attacks in Mumbai.

The information was handed over to domestic security services, according to an AP source. The official, who spoke on the condition of anonymity, said he didn't know which actions were taken next.

Additional security "could not have stopped what took place," Ratan Tata, chairman of the company that owns the Gran Taj hotel, told CNN. "They (the gunmen) didn't come through that entrance. They came from somewhere in the back."

The Indian government has been under intense pressure from protesters demanding answers about India's security apparatus following the terrorist strikes which killed almost 200 people. Critics say India's intelligence and security services failed to protect the country. Indian Prime Minister Manmohan Singh has promised to strengthen maritime and air security.

With files from The Associated Press