ISLAMABAD, Sept 21 (Reuters) - A suicide truck bomb exploded outside the Marriott Hotel in Islamabad on Saturday, killing at least 52 people in the deadliest attack to hit the Pakistani capital since Pakistan joined the U.S.-led war on terrorism in 2001.

Here are answers to the questions of who could be behind the blast and why it has been carried out now:

WHO DID IT?

Islamist militant groups linked to Al Qaeda and the Pakistani Taliban were almost undoubtedly behind the blast, though there has been no confirmed claim of responsibility.

They have been waging a campaign of violence ever since former army chief and ex-president Pervez Musharraf made a foreign policy U-turn, ditching support for the Taliban and jihadi groups to side with the United States after al Qaeda's Sept. 11, 2001 attacks on New York and Washington.

The militants intensified attacks across the country, killing hundreds of people, after a commando assault to crush an armed movement at Islamabad's Red Mosque in July last year.

WHY NOW?

The bombing happened in the wake of a series of U.S. missile strikes against militant targets in the Pakistani tribal lands bordering Afghanistan.

The militants believe the government and army have helped the United States, despite Pakistani protests following a U.S. commando raid, the first known operation by U.S. ground troops inside Pakistan, in the Waziristan tribal region on Sept. 3.

Hundreds of Islamist fighters have been killed in a major offensive by the Pakistani army in the Bajaur tribal region during the past few weeks.

Revenge is a powerful motive in the fiercely independent tribal region, where al Qaeda and the Taliban have laid down roots.

Hours before the attack, Pakistan's new president, Asif Ali Zardari, who was sworn in this month, told parliament that terrorism must be rooted out.

Zardari was due to go the United States on Sunday. He was expected to attend the U.N. General Assembly and meet President George W. Bush and Indian Prime Minister Manmohan Singh.

Analysts say the attack on the Marriott was meant to show the government and army what to expect if they support the United States.

WHY THE MARRIOTT?

The hotel is Pakistani-owned, but Marriott is a U.S. franchise. There is only one other five-star hotel in Islamabad.

The hotel is in a high-security zone close to the presidency building, the prime minister's residence and the parliament. Hitting the Marriott demonstrated the militants' ability to penetrate the security ring.

The hotel's clientele are members of the Pakistani elite, foreign diplomats, businessmen and journalists.

It has been targeted twice before. In January 2007 a security guard died stopping a suicide bomber from sneaking in through a side entrance hours before a reception hosted by the Indian High Commission. An earlier, small blast in the lobby area failed to kill anyone. (Writing by Zeeshan Haider and Simon Cameron-Moore; Editing by Kevin Liffey)

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