Kenya's military caused the collapse of three floors of Nairobi's Westgate mall during the terrorist siege in which at least 67 people died, a top-ranking government official has said.

The account comes before the release of findings from an ongoing forensic investigation into the attack and raises the possibility that the military may have killed hostages in their rescue attempt. An undisclosed number of people are reportedly buried in the rubble.

The official said autopsies will determine if any bodies found there were killed by falling debris or the attackers.

US ambassador Robert F Godec said the United States is concerned about the spectre of more violence from the Somali Islamic group, al-Shabaab, which claimed responsibility for the attack.

"Obviously they do pose a threat and it's critically important, I think, that we understand what the terrorists in that organisation are up to, how they carry out attacks and really seek to frankly end the threat that the organisation poses," Godec said. "We are working very hard with Kenya, and other countries, to do so."

FBI agents – along with investigators from Britain, Canada and Germany – have been dispatched to investigate the crime scene. Many of the dead were foreigners.

The Kenyan official also confirmed that Kenyan troops fired rocket-propelled grenades inside the mall, but would not say what was used to cause the collapse or whether it was intentional. The account at least partially backs up information from another official on Wednesday, who said rocket-propelled grenades fired by soldiers created a gaping hole in the mall's roof and caused the floors to collapse.

Inside the mall on Friday, a pushchair was overturned on the marble floor next to wilting fresh flowers at a florist shop. Slabs of concrete sat on top of flattened cars in the parking area while in other parts there are rows of scorched vehicles.

Four huge explosions rocked the mall on Monday. The government has not publicly explained what caused the floors to collapse. One official earlier suggested it was caused by a mattress fire inside the Nakumatt department store.

A destroyed section of the Westgate mall in Nairobi, Kenya. An undisclosed number of people are reportedly buried in the rubble. Photograph: AFP/Getty Images

Amid the possibility that some of the attackers escaped during the mass evacuation of civilians, Kenyan authorities have increased surveillance at border crossings and at Nairobi's airport, the senior official said.

A car has been discovered that is believed to have been used by the attackers who led the attack at the mall, the official added. Investigators are tracing the car's ownership after it was retrieved outside the mall.

Joseph Ole Lenku, Kenya's interior minister, said at least eight suspects are being held over the attack. Three others who had been detained were released.

Kenyan police have given little information since the attack, saying the investigation has only just begun.

It will take investigators at least seven days to comb through the rubble of the mall, where some bodies are believed to be buried, Lenku has said.

Al-Shabaab said it carried out the attack because Kenya sent its troops into Somalia to fight the militants.

The Kenyan Red Cross says 61 people remain missing and many worry their bodies may be buried in the destroyed part of the mall – though the government has insisted few victims are believed to still be inside.

The government says at least 67 people were killed in the assault by 12 to 15 al-Shabaab militants, including 61 civilians and six security forces. At least five attackers also were killed.