Presidential spokesman Manoah Esipisu said structural engineers are examining the collapse. FBI agents, along with investigators from Britain, Canada and Germany, are participating in the inquiry. Results are not expected until next week at the earliest.
Police are trying to determine if the attackers stored ammunition in the mall hours or even days before the attack, and investigators are tracing the ownership of a car that has been discovered and is believed to have been used by the gunmen.
Al-Shabab said it carried out Saturday’s attack to punish Kenya for sending its troops into Somalia to fight the al-Qaida-linked militant group that had seized large parts of that country for years before being dislodged from the capital, Mogadishu.
U.S. Ambassador Robert F. Godec said the United States is concerned about the specter of more violence from al-Shabab.
“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.”
Amid the possibility that some of the attackers may have escaped during the evacuation of civilians from the mall, authorities have increased surveillance at border crossings and at the Nairobi airport, the senior government official said.
Eight suspects are being held over the attack, Interior Minister Joseph Ole Lenku said. Three others who had been detained were released.
The government says at least 61 civilians and six security forces were killed. At least five attackers also were killed.
At the request of Kenya, Interpol on Thursday issued a notice asking for help in capturing 29-year-old British-born fugitive Samantha Lewthwaite — not in connection with the mall attack, but over a 2011 plot to bomb holiday resorts in Kenya.