Also Wednesday, President Uhuru Kenyatta announced three days of mourning.
While a government spokesman said the death toll was not expected to rise significantly from 67, an intelligence source told British newspaper The Guardian hundreds of people could remain buried in the ruins of a supermarket.
"There were 200 workers in Nakumatt and 800 people shopping. The walls of Nakumatt collapsed," the source said. "There are a lot of bodies inside."
The source also said at least one of the attackers was still alive and fighting in the ruins.
Al-Shabaab, the Somali militant group that has taken responsibility for the attack, accused Kenyan government forces of demolishing part of the mall. The group blamed the government for "burying" 137 hostages.
The government said a parking deck collapsed but denied deliberately bringing any part of the structure down. It also told The Guardian none of the attackers remain alive inside.
At a news conference, Lenku said experts from the United States, Germany, Britain, Canada, Israel and Interpol are helping with the investigation.
"This process involves fingerprinting, DNA and ballistics examination. We do not expect the numbers of the dead to increase in any significant way," he said. "Yes, there could still be bodies trapped in the rubble. We can only confirm that after we have gone through the rubble."
Kenyatta said the death toll included six security personnel, the BBC reported. Five al-Shabaab militants were killed and 11 suspects arrested, he said.
In his address, Kenyatta praised the response by ordinary Kenyans, calling it exemplary.
"We have [shamed] and defeated our attackers," he said. "Kenya has stared down evil and triumphed."
While not confirming reports that several of the attackers were American and British, the president said, intelligence reports "had suggested that a British woman and two or three American citizens may have been involved in the attack."
"We cannot confirm the details at present," he said. "Forensic experts are working to ascertain the nationalities of the terrorists. ...
"These cowards will meet justice, as will their accomplices and patrons, wherever they are."
The British Foreign Office said one Briton was arrested in Nairobi but did not provide more details, the BBC said.
The drama began Saturday when militants -- after reportedly escorting Muslims from the mall -- stormed into the tony shopping center and opened fire, CNN said. A senior Kenyan government official said the militants took "very few" people captive.
"They were not interested in hostage-taking," the official said. "They only wanted to kill."
"We wanted to negotiate," the Kenyan official said. "They didn't even respond."
At least 18 foreigners are among the dead, including six British citizens, as well as citizens from France, Canada, the Netherlands, Australia, Peru, India, Ghana, South Africa and China.
About 175 people were wounded.
Al-Shabaab, based in Somalia and linked to al-Qaida, said it carried out the attack in retaliation for Kenyan military operations in Somalia.
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