NEW DELHI Investigators began probing on Thursday the triple bomb blasts which killed 18 people in the financial capital Mumbai, in the biggest attack since Pakistan-based militants rampaged through the city in 2008.
No one has claimed responsibility for Wednesday's blasts, but security analysts say the pattern of the attack points to a local militant group called the Indian Mujahideen (IM).
Home Minister Palaniappan Chidambaram said it is too early to pinpoint the perpetrators, but the attacks could be in retaliation for a number of plots foiled by police or arrests, including of members from the Indian Mujahideen and Indian Maoists.
A remote possibility is the Pakistan-based separatist group Lashkar-e-Taiba (LeT), known for its sympathies for al Qaeda and blamed for the 2008 Mumbai attacks that killed 166 people.
Other groups that could have reasons to carry out the strike include groups fighting for independence of the Kashmir region, Maoist rebels or local mafia.
WHO ARE THE INDIAN MUJAHIDEEN?
The Indian Mujahideen is described by global intelligence firm Stratfor as "a relatively amateurish group that's been able to carry out low to medium intensity attacks".
While its members are mostly local Muslims, the group is suspected of having been trained and backed by militant groups in neighbouring Pakistan and Bangladesh.
The group first emerged during a wave of bombings in north India in 2007. They have since claimed responsibility for bomb attacks in the cities of Jaipur, Bangalore, Ahmedabad and New Delhi.
The last attack they claimed was in 2010 in the western city of Pune, where a bomb blast at a tourist spot killed nine people.
Police say the Indian Mujahideen may also include former members of Bangladeshi militant group Harkat-ul-Jihad al Islami.
The demands of the Indian Mujahideen, like their targets, have tended to be domestic. The group has declared "open war against India", accusing the Indian army of killing Muslims in Kashmir and also directing its ire at the Mumbai police anti-terrorist squad, accusing them of harassing Muslims.
WHO ARE LASHKAR-E-TAIBA?
The "army of the pure" is one of the largest Islamic militant groups in South Asia but has not been operating recently.
Once nurtured by Pakistan's military to fight India in Kashmir, it is now under a tight leash since the 2008 Mumbai attacks, for fear of a new attack that would invite retribution on Pakistan.
The group claimed responsibility for the attack on an army base in New Delhi's historic Red Fort which killed three people in late 2000 and for an assault on India's parliament in 2001 that brought India and Pakistan to the brink of a fourth war.
In 2005, it was blamed for bomb attacks on markets in New Delhi that killed more than 60 people.
The United States has designated the LeT as a "foreign terrorist organization". Pakistan banned it in 2002, but critics say it long operated openly under different names.
Mumbai is home to several powerful organised criminal gangs that run extortion, money laundering and smuggling rackets. They are active in real estate and in financing and distributing Bollywood films.
The biggest of these gangs, led by fugitive Dawood Ibrahim, has been accused of carrying out strikes on behalf of Pakistan-based militant and intelligence groups. Ibrahim's gang is blamed for a 1993 bomb attack in Mumbai that killed at least 230 people.
Ibrahim is believed by India to be in hiding in Pakistan. The United States in 2003 designated him as a "global terrorist" for his links to the al Qaeda and for financing militant groups.
His wealthy crime network is still believed to be active in India and capable of providing assistance for attacks, if not directly carry them out.
Maoists rebels control vast swathes of India's countryside, the so called "Red Corridor" that extends from the southern state of Andhra Pradesh through the central state of Chattisgarh and into West Bengal.
Their estimated 20,000 combatants have carried out hundreds of deadly attacks over the last few years, mostly on armed troops, and destroyed railway lines and factories in the hinterland.
The Maoists, who say they are fighting on behalf of poor peasants and indigenous tribes, have so far not attacked urban centres. It is doubtful whether they have the capability or intent to carry out such strikes.
WHY MUMBAI? WHY NOW?
India has long been under the threat of militant attacks by a variety of groups ranging from separatists in the northeast to Hindu nationalists and Islamists.
But there is a possibility the latest strike could be aimed at scuttling fledgling attempts to revive the peace process between New Delhi and Islamabad.
India and Pakistan have recently begun talks that were frozen after the 2008 Mumbai attacks and some of the progress has surprised observers.
But an attack linked to Pakistan will almost certainly put pressure on India to pull out of talks and take a hardline stance.
(Editing by Paul de Bendern and Sugita Katyal)