Islamist terrorist top the list of those who could be behind a series of bomb blasts in Mumbai that killed at least 183 people, according to security analysts.
The following is a list of possible suspects and motives, many of whom have links to each other.
* Lashkar-e-Taiba (Army of Taiba, the Islamic holy city of Medina): The prime suspect, Lashkar-e-Taiba, is thought to be one of the only groups with the organisation and the ruthlessness to carry out such large-scale attacks. But the Islamic terrorist group denied any role in what it called "inhuman and barbaric acts".
Lashkar is based in Pakistan and is fighting in Kashmir. Its operatives have been arrested recently along with arms and explosives in Maharashtra, of which Mumbai is the capital.
Lashkar could have been trying to undermine the peace process between India and Pakistan, fearing that Islamabad was ceding too much over Kashmir. It may also have wanted to provoke violence between majority Hindus and Muslims in India.
Lashkar was blamed for bomb attacks on markets in New Delhi last October that killed more than 60 people as well as a raid on the Indian parliament in December 2001 that brought India and Pakistan to the brink of a fourth war.
* Students Islamic Movement of India (SIMI): A banned group which is believed to have deep roots in many districts of Maharashtra and links to Lashkar, officials say.
In the past, Lashkar is known to have used SIMI activists to help carry out attacks. SIMI activists have been blamed for small train bombings in Uttar Pradesh in recent years.
* Other islamic terrorist groups: Jaish-e-Mohammed (Army of Mohammed) shared the blame with Lashkar for the 2001 attack on the Indian parliament. Jaish has been overshadowed by Lashkar since then but continues to operate in Kashmir with bases in Pakistan.
* Pakistan's Inter-Services Intelligence (ISI): New Delhi says Pakistan has not done enough to clamp down on Islamic militant groups using its territory to attack India. Some intelligence officials and analysts say Pakistan's powerful ISI continues to actively support such groups, to maintain pressure on India over disputed Kashmir.
Pakistan denies the charge.
* Al Qaeda: Tuesday's attacks were eerily reminiscent of bombings on commuter rail networks in Madrid and London in the past two years, which were linked to al Qaeda.
Analysts say Lashkar has links to al Qaeda and may either have been inspired by the bombings in Europe or have taken direct guidance from Osama bin Laden's organisation.
"India is now on the al Qaeda map," said security analyst and former Major-General Ashok Mehta, citing New Delhi's growing ties with Washington.
"This is a highly precise, well-planned operation with months of planning," he said, a hallmark of al Qaeda-inspired operations.
But Danny Yatom, former head of Israel's foreign intelligence service Mossad, told Israel Radio he did not believe al Qaeda was likely to be directly behind the attacks.
"After all, India has found itself facing a terrorist campaign for a long time, as a result of the conflict with Pakistan in Kashmir, and the Muslim separatists," he said.
*The Mumbai Muslim Underworld: Dawood Ibrahim, India's most wanted criminal and the leader of a powerful Muslim-dominated underworld gang, was blamed for the 1993 serial blasts in Mumbai that killed more than 250 people. Those attacks were believed to have carried out in revenge for the destruction of a Babri Majid in Ayodhya and resulting violence between Hindus and Muslims.
Ibrahim, who has been declared a "global terrorist" by the United States, is believed to be living in Pakistan.
But analyst Ashok Mehta said Ibrahim's organisation had been significantly weakened in recent years and Tuesday's attack was "out of his league".