Kenyan President Uhuru Kenyatta said security forces had defeated an al-Shabaab cell, holed up in the upscale Westgate shopping complex, after a bruising four-day siege that claimed at least 72 lives, including 61 civilians, six security officers and five “terrorists”, and injured another 175.

“Fellow Kenyans, we have been badly hurt and feel great pain and loss, but we have been brave, united and strong,” saidMr. Kenyatta in a televised address, “Kenya has stared down great evil and triumphed.”

The east African nation would observe three days of national mourning.

Mr. Kenyatta said several bodies were still trapped in the mall after the roof caved in, but made no reference to the hostages apparently held by the militants. He also did not state if the security forces had accounted for all the militants who participated in the attack.

The authorities have consistently refused to confirm the number of hostages captive in the mall, but senior officials frequently commented on the presence of hostages to explain why security operations were proceeding with great caution.

The Kenyan Red Cross confirmed that at least 62 people were missing as of Tuesday morning, amid speculation that many could still be trapped in the mall and the death toll could rise dramatically.

On Twitter accounts believed to be authentic, al-Shabaab spoke of “countless bodies still scattered inside the mall”, and this morning said, “hostages who were being held by the Mujahideen inside Westgate are still alive, looking quite disconcerted but, nevertheless, alive”. Kenyan authorities have dismissed such tweets as propaganda.

That morning, security forces intensified their operations, describing the escalation as a “final assault”. An eyewitness said he saw security forces fire mortars into the building and gunfire echoed around the complex.

On Tuesday afternoon, the National Disaster Operation Centre said Kenyan defence forces deployed at the mall had been replaced by the police force, indicating that the operation has shifted gears from fighting militants to securing the building. Intermittent explosions and gunfire continued, but authorities claimed Kenyan security forces caused the blasts as they secured the complex and defused explosives planted by the attackers.

People who had abandoned their vehicles outside the besieged mall complex when the fighting began on Saturday were asked to collect their vehicles, suggesting the authorities were confident that the danger had passed.

As the fighting winds down, the focus is shifting to the identity and motivations of the perpetrators.

Mr. Kenyatta said intelligence reports claimed a British woman and two or three American citizens might have participated in the attack, amidst media speculation that Samantha Lewthwaite, the so-called “White Widow”, British wife of London suicide bomber Germaine Lindsay, was involved in the atrocity.

In the city morgue in Nairobi on Tuesday, a team of American embassy officials dressed in hiking gear examined three unclaimed bodies but declined to explain their mission. An official at the morgue indicated the Americans suspected that some of the slain militants might be in the morgue, and said American investigators had checked fingerprints of the corpses.

“The Americans brought a machine to check the fingerprints with their database,” he said. The morgue, the official said, was preparing for a deluge of corpses that had lain in mall as the security forces battled the militants for four days.

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