Nairobi, Sep 26 (IANS/EFE): Kenyan police Wednesday began their investigation of the terrorist invasion of a Nairobi mall that left at least 72 people dead, including five of the assailants.

The probe, which is being assisted by the Federal Bureau of Investigation (FBI), Scotland Yard and the spy agencies of Israel and Germany, got under way as authorities searched for additional bodies in the rubble of the Westgate Mall.

For reasons that remain unclear, three of the structure's four floors collapsed during the battle between Kenyan security forces and the Muslim militants who stormed the mall Saturday.

Six Kenyan soldiers and 61 civilians were killed, while 62 other people remain hospitalised, President Uhuru Kenyatta said Tuesday in a televised address.

He warned, however, that the death toll could rise as trapped bodies are pulled from the wreckage, while the Red Cross said 71 people who may have been in the mall when the assault began remain unaccounted for.

No additional civilians are likely to be found in the rubble at Westgate, Kenya's interior minister told reporters Wednesday.

Any remaining bodies would be those of attackers, Joseph Ole Lenku said.

Al-Shabaab, the Somali-based Islamic militant group thought to be behind the attack, said Wednesday that 137 civilians died.

Kenyan police are looking into the possibility that some of the attackers rented space inside the mall in preparation for the terrorist strike.

Westgate is located in one of the most upscale areas of Nairobi and is frequented by the capital's Western expatriate community.

Al-Shabaab said it attacked the Nairobi mall in retaliation for the Kenyan military's 2011 incursion into Somalia.