Kenya’s military caused the massive 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 said Friday.

The account comes ahead of 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 had been slain earlier by the terrorists. The high-ranking police official spoke on condition of anonymity because he was not authorized to divulge sensitive information.

U.S. Ambassador Robert F. Godec said the United States is concerned about the specter of more violence from the Somali Islamic group, al-Shabab, which claimed responsibility for the attack.

“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.”

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 senior Kenyan police 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 given by 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.

Investigators, meanwhile, were piecing together a profile of a man who warned a pregnant woman to leave the mall just before the attack began on Saturday, the official said. The woman went home to safety. After her nephew wrote about her experience in a school essay on how he spent his weekend, his teacher informed police who have interviewed the woman about the mysterious man.

Inside the mall Friday, a baby stroller 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 Monday followed by dark smoke pouring from the shopping center, the likely time that the floors collapsed. 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.

Amid the possibility that some of the attackers escaped during the mass evacuation of civilians from the mall in Nairobi’s Westlands neighborhood, Kenyan authorities have increased surveillance at border crossings and at the Nairobi airport, the senior police official said.

No bodies have been retrieved from under the rubble since Kenya’s military secured the building on Tuesday, he said, adding that police are also investigating if the attackers stored ammunition inside the mall hours or even days before the attack.

A car has been discovered that is believed to have been used by the terrorists 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.

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