Nairobi - The militant group behind the four-day takeover of a Nairobi mall claimed Wednesday that Kenyan government forces used chemical weapons in their assault on the building, then carried out "a demolition" to cover evidence and buried 137 hostages.
In a series of Twitter posts from an account believed to be genuine, al-Shabaab said that "having failed to defeat the mujahideen inside the mall, the Kenyan govt disseminated chemical gases to end the siege".
They added "to cover their crime, the Kenyan govt carried out a demolition to the building, burying evidence and all hostages under the rubble".
Government spokesperson Manoah Esipisu immediately denied the claim, saying no chemical weapons were used and that the official civilian death toll remains 61.
"Al-Shabaab is known for wild allegations and there is absolutely no truth to what they're saying," he said. But officials said the death count will likely rise. Estimates varied between only a few bodies to dozens of bodies possibly still inside the mall.
A British man was arrested in Kenya following the terrorist attack, Britain's Foreign Office said.
The agency said in a statement on Wednesday that British officials are ready to provide assistance to the man. Officials would not provide his name or details. He is believed to be in his 30s. Britain's Daily Mail newspaper said he was arrested on Monday as he tried to board a flight from Nairobi to Turkey. The newspaper said his face was bruised and he was acting suspiciously.
Esipisu said floors of the mall collapsed after a fire started by the al-Shabaab attackers caused structural weakness in a 3rd floor parking lot, which then came down onto the second floor and brought it down onto the first, or ground floor.
He said there were known to be eight civilians in the rubble, which were included in the government's official death estimate. There could be several terrorists also buried, he said.
At the mall on Wednesday morning, gunshots could be heard. Esipisu said the shots came from Kenyan forces going room to room in the large Westgate Mall, firing protectively before entering unknown territory. President Uhuru Kenyatta told the nation the night before the terrorists had been defeated and declared three days of national mourning beginning on Wednesday.
"During sanitisation once you take control of the place if you go to a room where you haven't visited before you shoot first to make sure you aren't walking into an ambush," he said. "But there hasn't been any gunfire from the terrorists for more than 36 hours."
The Kenyan government said forensic experts from the United States, Britain and Israel would be assisting them in their investigation of the attack.
"The mall is sealed off. It is a crime scene," Esipisu said.
The process of retrieving bodies from inside the mall still had not begun on Wednesday morning — possibly indicating that the situation was not yet considered secure — though a city morgue official said his workers were preparing to go into the building soon.
The attack claimed by Somali militant group al-Shabaab killed at least 61 civilians, six security officers and five extremists, the president said.
Another 175 people were injured, including more than 60 who remain hospitalised.
Fears persisted that some of the attackers could still be alive and loose inside the rubble of the mall, a vast complex that had shops for retailers like Bose, Nike and Adidas, as well as banks, restaurants and a casino.
Major terror attack
A high-ranking security official involved in the investigations said it would take time to search the whole mall before declaring that the terrorist threat had been crushed. That official insisted on anonymity in order to discuss information not publicly disclosed.
Eleven other suspects have been taken into custody, and Esipisu said: "At this at this point the interrogations are ongoing and I can't reveal any of the details."
Al-Shabaab, whose name means "The Youth" in Arabic, first began threatening Kenya with a major terror attack in late 2011, after Kenya sent troops into Somalia following a spate of kidnappings of Westerners inside Kenya.
The al-Shabaab extremists stormed the mall on Saturday, throwing grenades and firing on civilians.
The group used Twitter to say that Somalis have been suffering at the hands of Kenyan military operations in Kenya, and the mall attack was revenge.
"You could have avoided all this and lived your lives with relative safety," the group Tweeted on Tuesday. "Remove your forces from our country and peace will come."
The militants specifically targeted non-Muslims, and at least 18 foreigners were among the dead, including six Britons, as well as citizens from France, Canada, the Netherlands, Australia, Peru, India, Ghana, South Africa and China. Five Americans were among the wounded.
The mall attack was the deadliest terrorist attack in Kenya since the 1998 al-Qaeda truck bombing of the US Embassy in Nairobi, which killed more than 200 people.
Security officials in Nairobi always knew that Westgate, which was popular with foreign residents of the capital as well as tourists and wealthy Kenyans, was a likely target for terror attacks.