NAIROBI, Kenya (AP) -- Kenya's military caused the collapse of three floors of the Westgate Mall in the deadly terrorist siege, a top-ranking official disclosed Friday, while the government urged patience with the pace of an investigation that has left key questions unanswered.
Seven days after 67 people were killed in the attack on the upscale shopping center, there is still no clear word on the fate of dozens who have been reported missing and no details on the terrorists who carried it out.
The account of the roof collapse raises the possibility that the military may have caused the death of hostages in its rescue attempt. An undisclosed number of people are feared to be buried in the rubble.
The official said autopsies will be conducted on any bodies found to determine the cause of death -- from the militants or the structural collapse. The high-ranking government official spoke to The Associated Press on condition of anonymity because he was not authorized to divulge sensitive information.
The official also confirmed that Kenyan troops fired rocket-propelled grenades inside the mall, but would not say what caused the floors to collapse, if the action was intentional, or if it was an accident.
The account at least partially backs up information given to AP on Wednesday by another official who said RPGs fired by soldiers created a gaping hole in the mall's roof and caused the floors to collapse.
Four huge explosions had rocked the mall Monday and dark smoke poured out -- the likely time that the floors collapsed.
A soldier who was returning from the mall Tuesday while carrying a rocket launcher told the AP reporter that he had fired it inside. The soldier spoke on condition of anonymity because he was ordered not to talk to the media.
The government has not said publicly what caused the collapse. One official had earlier suggested it was caused by a mattress fire in the Nakumatt department store.
Presidential spokesman Manoah Esipisu said structural engineers are examining the collapse. FBI agents, along with investigators from Britain, Canada and Germany, are participating in the inquiry. Results are not expected until next week at the earliest.
Police are trying to determine if the attackers stored ammunition in the mall hours or even days before the attack, and investigators are tracing the ownership of a car that has been discovered and is believed to have been used by the gunmen.
Al-Shabab said it carried out Saturday's attack to punish Kenya for sending its troops into Somalia to fight the al-Qaida-linked militant group that had seized large parts of that country for years before being dislodged from the capital, Mogadishu.
U.S. Ambassador Robert F. Godec said the United States is concerned about the specter of more violence from al-Shabab.
"Obviously they do pose a threat, and it's critically important, I think, that we understand al-Shabab, understand what the terrorists in that organization are up to, how they carry out attacks, and really seek to frankly end the threat that the organization poses," Godec said in an interview with AP. "So we are working very hard with Kenya, and other countries, to do so."
Amid the possibility that some of the attackers may have escaped during the evacuation of civilians from the mall, authorities have increased surveillance at border crossings and at the Nairobi airport, the senior government official said.
Eight suspects are being held over the attack, Interior Minister Joseph Ole Lenku said. Three others who had been detained were released.
The government says at least 61 civilians and six security forces were killed. At least five attackers also were killed.
At the request of Kenya, Interpol on Thursday issued a notice asking for help in capturing 29-year-old British-born fugitive Samantha Lewthwaite -- not in connection with the mall attack, but over a 2011 plot to bomb holiday resorts in Kenya.
Known in British tabloids as "the white widow," she was married to one of the suicide bombers in the 2005 attack on London's transit system, and British media have speculated she was involved in the mall attack.
But Lenku suggested that Interpol raised the issue of the international alert. He declined to discuss any links to the mall attack, saying he "wouldn't want to pre-empt the position of the forensic investigation taking place."
Dramatic new video filmed by the Kenyan Red Cross shows responders rushing to help the injured shortly after the assault began on Sept. 21.
The footage, obtained by The Associated Press shows responders entering the mall's upper level parking area and treating injured amid bodies on the pavement.