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 andat 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