Somali's rebel group al-Shabab vowed in late 2011 to carry out a large-scale attack in Nairobi in retaliation for Kenya's sending of troops into Somalia to fight the Islamic insurgents.
Off duty Sgt. Major Frank Mugungu said Saturday he saw four male attackers and one female, and that he could clearly identify one of the gunmen as a Somali, though he could not identify the rest.
A self-proclaimed spokesperson for al-Shabab tweeted indicating the group's involvement.
"The Kenyan government, however, turned a deaf ear to our repeated warnings and continued to massacre innocent Muslims in Somalia #Westgate," @HSM_Press tweeted.
A CIA official told ABC News: "We've seen the al-Shabab claim but don't have anything solid yet to confirm that claim or not."
Another witness said the attackers were speaking Swahili, the language of Kenya and other East Africans.
It appears the attack began at the outdoor seating area of Artcaffe at the front of the mall, witnesses said.
"We started by hearing gunshots downstairs and outside," Patrick Kuria, an employee at Artcaffe, said. "Later we heard them come inside. We took cover. Then we saw two gunmen wearing black turbans. I saw them shoot."
Some people were shot at the entrance to the mall after volleys of gunfire moved outside and a standoff with police began. Ambulances continued to stream in and out of the mall area, ferrying the wounded who gradually emerged from hiding inside the mall.
The Associated Press contributed to this report.