"They are checking for any potential explosive devices left behind," a security source said, adding that specialist remote-controlled demining robots were on hand.
An AFP reporter outside the bullet-riddled mall also saw teams of sniffer dogs, which will check not only for explosives, but for the grim expectation of the bodies of over 60 missing victims.
President Uhuru Kenyatta announced late Tuesday the 80-hour siege was over, with the "immense" loss of 61 civilians and six members of the security forces.
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However, police said the current death toll was provisional, while the Kenyan Red Cross said 63 people were still listed as missing.
The president said "three floors of the mall collapsed, trapping several bodies within the rubble including those of terrorists."
Somalia's Al Qaeda-linked Shebab rebels said the group carried out the attack in retaliation for Kenya's two-year battle against the extremists' bases in the country.
Five attackers were killed and 11 suspects detained.
In one of the worst attacks in Kenya's history, the militants marched into the four-storey, part Israeli-owned Westgate Mall at midday Saturday, spraying shoppers with automatic weapons fire and tossing grenades.
Kenyatta said that "forensic investigations are under way to establish the nationalities of all those involved" amid reports Americans and a British woman were among the insurgents.
There has been growing media speculation at the possible role of wanted British extremist Samantha Lewthwaite, daughter of a British soldier and widow of suicide bomber Germaine Lindsay, who blew himself up on a London Underground train on July 7, 2005, killing 26 people.
The president said intelligence reports had suggested that a British woman and two or three American citizens "may have been involved in the attack", but that could not yet be confirmed.
