(CBS News) NAIROBI, Kenya - Flags flew at half mast Wednesday as Kenya began three days of mourning for the victims of the massacre at Nairobi's Westgate Mall, as details began to emerge about the final assault against the militants from the al Qaeda-linked Somli militant group al-Shabab.
CBS News correspondent Charlie D'Agata reported that there was a sense of relief in Nairobi that the ordeal was over, but the search continued for bodies inside the mall, and one of the few things that was clear, was that none of the remaining hostages made it out alive.
New video emerged showing the brave actions of a plain-clothes Kenyan police officer who made sure a mother and her children did not suffer a similar fate at the hands of the gunmen. The video came from the first hours of the terrorists' assault on the up-market mall in Kenya's capital, which was popular with foreigners.
Kenyan authorities reportedly tried to negotiate, but al-Shabab militants didn't even respond.
There are also reports that as a helicopter hovered overhead, gunmen started executing hostages and tossing their bodies off the roof.
Any planned rescue operations were further hampered Tuesday by smoke from fires set by the gunmen inside.
President Uhuru Kenyatta said Kenya "stared down evil and triumphed."
"We have ashamed and defeated our attackers," he said Tuesday. "That part of our task has been completed by our multi-agency security teams."
He said three floors of the mall collapsed and several bodies were buried beneath the rubble, including those of terrorists and hostages.
Al-Shabab, whichunfolding in the mall in almost real time via a Twitter account, claimed Wednesday that the Kenyan forces had "disseminated chemical gases to end the siege." They did not specify what "gases," and Kenyan authorities denied that any chemical agents had been used.
The terror group also claimed that 137 people were killed in the siege, but that number could not be independently confirmed, and the Kenyan government had yet to issue a final death toll. According to officials, the death toll stood at 61, as it was stated on Tuesday.
The sheer amount of firepower used over the course of the four-day siege has led Kenyan investigators to chase reports that some of the attackers had rented a shop in the mall three months prior to the attack, enabling them to sneak in and stock up on weapons and ammunition.
That first wave of the massacre left at least 60 people dead and 175 wounded.
President Kenyatta has said intelligence reports suggested two or three Americans might have been involved in the attack, but he also said forensic experts would have to examine the bodies of the attackers to confirm their identities.