NAIROBI, Kenya — Kenya's military caused the collapse of three floors of the Westgate Shopping Mall in the deadly terrorist siege, a top-ranking official has disclosed.

The Kenya goverment, meanwhile, is urging patience with the pace of an investigation that has left key questions unanswered.

One week after at least 67 people were killed in the attack on the upscale shopping center, there is still no clear word on the fate of dozens who were 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 and whether the action was intentional or 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 whether the attackers stored ammunition in the mall hours or even days before the attack, and investigators are tracing the ownership of a car that had 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-Qaeda-linked militant group that had seized large parts of that country for years before being dislodged from the capital, Mogadishu.

U.S. Ambassador Robert 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, 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 the injured amid bodies on the pavement.

The end of the graphic video appears to show several seriously wounded and dead people where a children's cooking event had been taking place. Injured people are wailing and calling out for help.

Inside the mall Friday — the last of three days of an official mourning period — a baby stroller could be seen overturned on the marble floor next to wilting fresh flowers at a florist's shop. Slabs of concrete sat atop flattened cars in the parking area. Elsewhere, there were rows of scorched vehicles.

Kenyan officials have offered at times contradictory accounts of the siege and are reluctant to release many details of the investigation prematurely.

Lenku urged patience, saying the investigation is "a very delicate and complex operation that requires time."

He said no bodies had been recovered from the rubble and no official reports of missing persons had been filed. The Kenyan Red Cross has said 59 people are unaccounted for, raising concern that there are still bodies in the debris.

The possibility that government troops were behind the roof collapse only added to the frustrations of Kenyans questioning what happened at the mall.

"You have to blame the government, not the soldiers, because they had to get their orders from somewhere," said John Odera, a security guard. "They should be held responsible for what they did."

A user-generated list of more than 80 questions on social media includes: "How many terrorists were involved in the attack?" ''Are any terrorists loose in the city?" ''Can we see the bodies of the 'neutralized' terrorists?" and "Are we still safe?"

The list reflects a broader frustration in Kenya over the lack of information about the attack.

Boniface Mwangi, a photographer who has emerged as one of Kenya's leading political activists, said he believes the lack of a clear accounting is due to the government trying to paper over shortcomings in its handling of the operation.

"They're trying to cover up something," he said. "If it's true you have nothing to hide, let's know what really happened."