ISLAMABAD, Pakistan (CNN) -- Pakistan's president, prime minister and other leaders had planned to dine at the Marriott Hotel in Islamabad on Saturday night, when it was struck by a massive truck bombing, a government official said Monday.
The hotel was destroyed by the 600 kg bomb.
The Saturday night suicide truck attack killed 57 people and wounded more than 250, and sparked a fire that left the hotel in ruins.
Al-Arabiya TV reported Monday that it received a claim of responsibility for the attack by a previously unknown group called "The Fedayeen of Islam," but noted that it could not authenticate the audio recording or the name of the group.
According to the recording, the group said 250 U.S. Marines and other U.S. and NATO officials were inside the hotel at the time of the attack.
It said the group regretted the attack, but said it was necessary to press its demands, including an end to U.S.-Pakistani joint efforts and a halt to all military operations in Pakistan's tribal regions.
The group also called for the release of "mujahedeen" prisoners in U.S. prisons, according to the recording. CNN cannot independently verify the claim.
The head of Pakistan's Interior Ministry, Rehman Malik, said earlier Monday that the government officials' dinner plans were changed at the last minute, when President Asif Ali Zardari asked to move the event to the prime minister's compound.
Malik addressed reporters during a handover service for Czech Ambassador Ivo Zdarek, who died in the blast.
His announcement raised questions as to how much the government knew about the planned attack, which involved a construction truck with more than 600 kg (1,300 pounds) of explosives. Watch the damage caused and hear stories from survivors »
But the hotel owner denied the government had made reservations for that night and Pakistani Senator Javed Ashraf Qazi said he was invited to the dinner but it was always scheduled to be at the prime minister's office.
A U.S. State Department contractor is still unaccounted for in the aftermath of the blast, State Department spokesman Robert Wood said Monday.
Two American military personnel who worked for the U.S. Embassy in Islamabad were among those killed, the U.S. military said. The Pentagon on Monday named an airman killed as Maj. Rodolfo I. Rodriguez, 34, of El Paso, Texas. A Lithuanian Pakistani was also among the fatalities, police superintendent Sheikh Zubair told CNN Sunday. The injured included 11 foreigners, Malik said
On Sunday Malik called the massive blast "the biggest attack, volume-wise" in Pakistan in seven years, based on the quantity and type of explosives used.
No arrests have been made in connection with the attack. But Malik said suspicion is falling on militants in Pakistan's tribal regions.
"I am not in a position to tell you who has done it, but (in) all the previous investigations, all the roads have gone to South Waziristan," he said Sunday.
But Amir Mohammad, an aide to leader of the Pakistani Taliban Baitullah Mehsud, said he shared the country's grief and was not involved, The Associated Press reported.
Saturday's massive blast left a nearly 18-meter-wide (60 foot) crater, which was 7m deep (24 feet), Malik said. It also caused a natural gas leak that set the top floor of the five-story, 258-room hotel on fire, police said. The blaze quickly engulfed the entire structure.
The blast occurred about 7:50 p.m., after the breaking of the fast during the holy month of Ramadan, Malik said. Trees were felled by the explosion, which occurred hours after newly elected President Zardari addressed a joint session of parliament and promised to root out terrorism.
At a news conference in Islamabad on Sunday, Pakistani authorities released security video of the blast, showing a small explosion inside the truck before the larger, deadly explosion.
In the video, a large truck crashes into the security gate, sending one security officer scurrying for safety. Then, as security guards approach the truck, the top of the vehicle explodes and the security guards flee. Watch guards scatter after an explosion »
A small cloud of smoke appears above the truck, which is engulfed in flames minutes later. One of the security guards tries to put out the fire with a hand-held extinguisher, to no avail. The guards then walk away, and the camera freezes on the burning truck. Watch the truck bomb caught on tape »
Pakistani officials said the blast apparently disrupted electricity to the area, causing the closed-circuit television camera to malfunction.
The Marriott, a Western brand-name hotel, has been the site of attacks in the past.
Malik said authorities had received a threat against the parliament two days ago. "We had taken all security measures," he said. "There was heavy security in the city."
Located near the diplomatic section of the city and heavily guarded by police and military, the hotel is popular among tourists and had been packed Saturday night. See where the attack occurred »
• Nine Pakistani soldiers were killed and six injured when a suicide car bomber crashed into a security checkpoint Monday night in Pakistan's Swat District of the North West Frontier Province, a Pakistani military spokesman said.
• Militants kidnapped an Afghan diplomat and killed his driver in northwest Pakistan Monday, police said. The diplomat, Abdul Khaliq Farahi -- who works in the Afghan consulate in Peshawar -- was on his way home when someone opened fire, police in Peshawar said.
CNN's Zein Basravi and Dan Rivers contributed to this report
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