The BBC, citing "a senior security source," said the shop would have made a good cover, giving the militants access to service elevators. That would have allowed them to smuggle arms, ammunition and explosives into the mall.
Members of al-Shabaab, an al-Qaida-affiliated group based in Somalia, would have needed false identification to get a lease in the mall, possibly obtained by bribing officials, the BBC said. Westgate was one of Nairobi's premier shopping centers and supposed to be under tight security.
"Terrorism is an exploitation of openings," Ndung'u Gethenji, chair of Kenya's parliamentary defense committee, told the BBC.
At least 67 people were killed in the attack that began last Saturday, including a nephew of President Uhuru Kenyatta and his fiancee. Kenyatta attended their funeral Friday at a Nairobi church, giving a eulogy.
Al-Shabaab has alleged Americans were involved in the attack but U.S. investigators in Kenya said they have not found any evidence to support the allegations, NBC News reported.
"We don't have any verifiable information that would indicate one way or the other way that Americans were involved," U.S. Attorney General Eric Holder said Thursday.
Interpol issued a "wanted" alert for Samantha Lewthwaite, also known as "Sherafiya" and "the White Widow," a British woman believed to be a key member of al-Shabaab.
She is wanted in Kenya on charges of possession of explosives and conspiracy to commit a felony dating back to December 2011, Interpol said.