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.
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-Qaida.
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 Osama bin Laden and al-Qaida 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-Qaida strike, homegrown Taliban militants probably had learned how to execute an attack of such magnitude.
Al-Qaida 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-Qaida 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 suicide 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 suicide 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-Qaida took responsibility.