Taliban militants based near the Afghan border and their Al Qaeda allies are the most likely suspects behind a massive truck bombing at Islamabad's Marriott Hotel, officials and experts said Sunday. At least 53 died in the explosion, including two U.S. Defense Department employees and the Czech ambassador.

The truck sat burning and disabled at the hotel gate for at least 3 1/2 minutes as nervous guards tried to douse the flames before they, the truck and much of the hotel forecourt vanished in a fearsome fireball on Saturday night, according to dramatic surveillance footage released Sunday.

The attack on the American hotel chain during Ramadan, among the deadliest terrorist strikes in Pakistan, will test the resolve of its pro-Western civilian rulers to crack down on growing violent extremism which many here blame on the country's role in the U.S.-led war on terror.

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While no group has claimed responsibility, the scale of the blast and its high-profile target were seen by many as the signature of media-savvy Al Qaeda.

Interior Ministry chief Rehman Malik said "all roads lead to FATA" in major Pakistani suicide attacks — referring to Federally Administered Tribal Areas, where U.S. officials worry that Usama bin Laden and Al Qaeda No. 2 Ayman al-Zawahri are hiding.

Mahmood Shah, a former government security chief for Pakistan's tribal areas, said that while the attack had "all the signatures" of an Al Qaeda strike, homegrown Taliban militants probably had learned how to execute an attack of such magnitude.

Al Qaeda was providing "money, motivation, direction and all sort of leadership and using the Taliban as gun fodder," he suggested.

A Pakistani intelligence official, speaking on condition of anonymity because he is not authorized to speak on the record to media, said investigators were examining just that theory.

Pakistani Prime Minister Yousuf Raza Gilani said the attack was an attempt to "destabilize democracy" in Pakistan, which this year emerged from nine years of military rule, and destroy its already fragile economy.

Gilani also claimed that the bomber attacked the hotel only after tight security prevented him from reaching Parliament or the prime minister's office, where President Asif Ali Zardari and many dignitaries were gathered for dinner.

However, the owner of the hotel accused security forces of a serious lapse in allowing a dump truck to approach the hotel unchallenged and not tackling the driver more clinically.

"If I were there and had seen the suicide bomber, I would have killed him. Unfortunately, they didn't," Sadruddin Hashwani said.

The bomb went off close to 8 p.m. Saturday, when the restaurants inside would have been packed with Muslim diners breaking their daily fast during the holy month of Ramadan.

The explosion wrecked a favorite spot for foreigners as well as the Pakistani elite that has been targeted twice before by militant bombings. The building — one of the few places outside the diplomatic district where U.S. diplomats were permitted to socialize — was still smoldering 24 hours after blast, which also wounded more than 260 people.

Anti-American feeling is running particularly high following a series of strikes by U.S. forces based in Afghanistan on Islamic militants nested in Pakistan's tribal belt.

U.S. Embassy spokesman Lou Fintor said there was no evidence that Americans were the target.

Still, he confirmed that two Defense Department employees were among the dead and that a third American — a State Department contractor — was missing.

Three U.S. Embassy employees and an embassy contractor were injured, Fintor said.

IntelCenter, a group which monitors and analyzes extremist communications, said senior Al Qaeda leader Mustafa Abu al-Yazid threatened attacks against Western interests in Pakistan in a video timed to the recent anniversary of the Sept. 11, 2001, attacks in the United States.

Malik, the interior minister, declined a reported offer of U.S. assistance in the investigation, saying Pakistani agencies could cope.

Rescue teams searched the blackened hotel room by room Sunday, finding several more bodies, and Gilani said the death toll had risen to 53. A Danish diplomat was also listed as missing and rescue workers said they expected to find more human remains.

Officials confirmed that Czech Ambassador Ivo Zdarek was also among the dead. Zdarek, 47, only moved to Islamabad in August after four years as ambassador to Vietnam.

Malik said one Vietnamese citizen was also killed. The wounded also included Britons, Germans, and several people from the Middle East.

Malik told a news conference that the bomb contained an estimated 1,300 pounds of military-grade explosives as well as artillery and mortar shells and left a crater 60 feet wide and 24 feet deep in front of the main building.

The government released footage from a hotel surveillance camera showing the heavy truck turning left into the gate at speed, ramming a metal barrier and jolting to a halt about 60 feet away from the hotel.

Guards nervously came forward to look, then scattered after an initial small explosion.

Several guards tried repeatedly to douse flames spreading through the cab of the truck as traffic continued to pass on the road behind. There was no sign of movement in the truck and the footage played didn't show the final blast.

Officials said vehicles carrying construction materials are allowed to move after sunset, meaning the sight of a dump truck near the government quarters might not have aroused suspicion.

The bombing came just hours after Zardari made his first address to Parliament since becoming president, less than a mile away from the hotel.

It drew condemnations from around the world, including from Bush, whose administration has pressured Pakistan to do more to put more pressure on militants using Pakistani soil to support the increasingly deadly insurgency in Afghanistan.

A recent series of suspected U.S. missile strikes and a rare American ground assault in Pakistan's northwest have signaled Washington's impatience with Pakistan's efforts to clear out militants. But the cross-border operations have drawn protests from the Pakistani government, which warned they would fan militancy.

The Marriott blast could prompt diplomats and aid groups in Islamabad to re-evaluate whether nonessential staff and family members should stay. U.N. officials met Sunday to discuss the security situation and, for now, made no decision to change their measures, said Amena Kamaal, a spokeswoman.

Zardari, who on Sunday was headed to New York to lead a delegation to the United Nations and was expected to meet with Bush during the week, spoke out against the cross-border strikes in his speech to Parliament. He condemned the "cowardly attack" afterward in an address to the nation.

Pakistan's powerful army chief, Gen. Ashfaq Parvez Kayani, joined the condemnation Sunday, calling the attack "heinous" and saying the army stands "with the nation in its resolve to defeat the forces of extremism and terrorism."

The army has staged offensives against insurgents in the nation's northwest that have drawn revenge attacks by Taliban militants.

The country's deadliest homicide bombing was on Oct. 18, 2007, and targeted ex-Prime Minister Benazir Bhutto — Zardari's wife — who survived. It killed about 150 people in Karachi during celebrations welcoming her home from exile.

Bhutto was assassinated in a subsequent attack on Dec. 27, 2007.

The last big attack in Islamabad was a homicide car bombing in June outside the Danish Embassy that killed six people in apparent revenge for the publication in Denmark of cartoons depicting the Prophet Muhammad. Al Qaeda took responsibility.

The names of the service members are being withheld pending notification of the next of kin and release by the Department of Defense.

Interior Ministry chief Rehman Malik said Ivo Zdarek, the Czech ambassador to Pakistan, was also among the dead.

The hotel, a favorite spot for foreigners and the Pakistani elite — and a previous target of militants — still smoldered from a fire that raged for hours after the previous day's explosion, which also wounded at least 250 people.

The targeting of the American hotel chain was one of the largest terrorist attacks ever in Pakistan and came at a time of growing anger in Pakistan over a wave of cross-border strikes on militant bases by U.S. forces in Afghanistan. Two Americans was killed and several were wounded.

The bomb went off at close to 8 p.m., when four restaurants inside would have been packed with diners at the hour that Muslims break their daily fast during the holy month of Ramadan.

"I didn't understand what it was, but it was like the world is finished," said Mohammad Sultan, a hotel employee who was in the lobby when the bomb exploded.

After the attack, rescuers ferried a stream of bloodied bodies from the gutted structure and the fire was still burning more than six hours after the blast, which sent up a thick pall of smoke over the area.

The explosion left a vast crater some 30 feet deep in front of the building.

Investigators on Sunday examined the gaping hole for evidence and rescue teams searched the building room by room. But the temperatures remained high and fires were still being put out in some parts.

Officials said the main building could still collapse.

"The building's structure is dangerous," said Malik Ashraf Awan, a senior civil defense officer. "It consumed too much heat and shock."

Prime Minister Yousuf Raza Gilani told reporters Sunday that the death toll had reached 53.

The bombing came just hours after President Asif Ali Zardari made his first address to Parliament, less than a mile away from the hotel, and days ahead of the new leader's meeting with President Bush on Tuesday in New York on the sidelines of the U.N. General Assembly.

Malik told The Associated Press it was unclear who was behind the attack and there had been no claim of responsibility. But authorities had received intelligence that there might be militant activity linked to Zardari's address to Parliament and security had been tightened, he said.

Analysts said they suspect the attack was a warning from Islamic extremists to the new civilian leadership of Pakistan that it should end already-strained cooperation with the United States to pursue Al Qaeda and Taliban militants entrenched in the lawless tribal region along the border with Afghanistan.

Terrorism researcher Evan Kohlmann said the attack was almost certainly either Al Qaeda or Pakistani Taliban.

"We are looking at either Al Qaeda or Tehrik-e-Taliban (Pakistan)," he said. "It seems that someone has a firm belief that hotels like the Marriott are serving as 'barracks' for western diplomats and intel personnel, and they are gunning pretty hard for them."

According to the U.S.-based IntelCenter, an Al Qaeda video released to mark the anniversary of the Sept. 11, 2001 attacks in the United States threatened attacks against Western interests in Pakistan.

The threat was made by senior Al Qaeda leader Mustafa Abu al-Yazid, who claimed responsibility for the summer bombing of the Danish Embassy in Islamabad.

Zardari reappeared after midnight on state television to condemn the "cowardly attack." He said he understood the victims' pain because he had buried his own wife — assassinated former Prime Minister Benazir Bhutto — in December.

"Make this pain your strength," he said. "This is a menace, a cancer in Pakistan which we will eliminate. We will not be scared of these cowards," he said.

Witnesses and officials said the dump truck exploded about 60 feet away from the hotel at two heavy metal barriers blocking the entrance. The location of the hotel made it vulnerable. It lies just off a busy thoroughfare, less than a mile from the presidential offices and Parliament. The security gate was well within the range of the blast wave.

The explosion reverberated throughout Islamabad and shattered windows hundreds of yards away.

Mohammed Asghar, a worker from a nearby office with a makeshift bandage round his head, said there was more than one man in the truck and that they had argued with the hotel guards.

"Then there was a flash of light, the truck caught fire and then exploded with an enormous bang," he said.

Senior police official Asghar Raza Gardaizi estimated the truck carried more than 2,200 pounds of explosives. He said in the midst of the rescue operation that at least 40 people were killed and many more feared buried in the rubble.

However, Kamal Shah, a senior Interior Ministry official, said earlier Sunday he knew of only 38 confirmed deaths.

Associated Press reporters saw at least nine bodies scattered at the scene. Scores of people, including foreigners, were running out — some of them stained with blood.

Information Minister Sherry Rehman said at least 250 people were wounded. Hospital staff and other officials said 21 foreigners were among the injured, including two Americans.

Pakistan faces a raging insurgency by the Taliban in the tribal regions along the border with Afghanistan, where Western governments worry that Al Qaeda militants could be plotting more attacks on their cities. Security officials say the two groups work together to carry out attacks in Afghanistan and Pakistan.

Officials have warned that increasing U.S. cross-border raids on Pakistani territory recently could fuel violent extremism.

President Bush said the attack was "a reminder of the ongoing threat faced by Pakistan, the United States, and all those who stand against violent extremism."

State Department spokesman Sean McCormack said at least one American was killed and several were wounded. "This barbaric attack comes during the month of Ramadan, only underscoring that those responsible have no respect for the principles of their faith," he said.

U.N. Secretary-General Ban Ki-moon strongly condemned the "heinous terrorist attack," and said "no cause can justify the indiscriminate targeting of civilians."

U.S. Embassy spokesman Lou Fintor said officials were trying to account for embassy staff and any other Americans affected.

Ambulances rushed to the area, picking their way through the charred carcasses of vehicles that had been in the street outside. Windows in buildings hundreds of yards away were shattered. Tropical fish from the tanks inside lay among the torn furnishings in the entrance area.

The hotel served as the headquarters for the international media during the 2001 war against the Taliban in neighboring Afghanistan. Its 290 rooms and suites and popular health club stood in a plot surrounded by government office buildings less than a mile from the president's office and Parliament. It had been targeted before.

In January 2007, a security guard blocked a homicide bomber who triggered a blast just outside the Marriott, killing the guard and wounding seven other people.

The country's deadliest homicide bombing was on Oct. 18, 2007 and targeted Bhutto, who survived. It killed some 150 people in Karachi during celebrations welcoming her home from exile. Bhutto was assassinated in a subsequent attack on Dec. 27.

On Aug. 21, 2008, homicide bombers blew themselves up at two gates into mammoth weapons factory in town of Wah, killing at least 67 people and wounding more than 70.

The Associated Press contributed to this report.