* Kenyan president says militants "defeated"
* Somali group says Kenyan troops used gas, denied by Kenya
* Unknown number of bodies under rubble of collapsed floors
* Al Shabaab demanded Kenyan troops leave Somali
Bomb disposal experts and investigators searched through the wreckage of a Kenyan shopping mall on Wednesday after a four-day attack by Islamist militants that killed at least 72 people.
President Uhuru Kenyatta declared three days of mourning after troops defeated the al Qaeda-linked al Shabaab group that targeted the upscale shopping centre popular with prosperous Kenyans and foreigners.
The militants stormed the mall, known for its Western shops selling iPads and Nike shoes, in a hail of gunfire and grenades on Saturday lunchtime. The attack ended on Tuesday when Kenyan troops set off a series of explosions inside the building.
Kenyatta said five militants and six security personnel were killed and 61 civilians had so far been confirmed dead but an unknown number of corpses are buried under the masonry.
Three floors collapsed after the blasts and a separate fire weakened the structure of the vaulted, marble-tiled building. Officials said the blaze was due to militants lighting mattresses as a decoy.
"Forensic investigators are on the site now," said a senior official from the National Disaster Operations Centre, speaking near the mall and adding that foreign agents were on the scene. He did not identify the agents.
Al Shabaab, which said it launched the assault to demand Kenya withdraw its troops fighting with African peacekeepers in Somalia, said hostages were killed when Kenyan troops used gas to clear to the mall. Officials dismissed this as "propaganda".
Kenyatta has said Kenyan forces would not quit Somalia.
"We have ashamed and defeated our attackers," he said in a televised address on Tuesday.
Israel has sent advisers to help the search, according to an Israeli source. The United States also has Federal Bureau of Investigation personnel on the ground. Others countries including Britain have offered help. Several foreigners have been listed among the dead.
The attack has highlighted the reach of the Somali group and the capabilities of its crack unit believed to be behind the attack, confirming Western and regional fears that as long as Somalia remains in turmoil it will be a recruiting and training ground for militant Islam.
"The bodies are still lying there in the rubble. We don't know how many exactly," said the NDOC official.
"The investigators will be looking to see what information they can extract to identify the terrorists and their nationalities, including DNA tests," he said, after Kenyan officials said the attack was a "multinational" operation.
Eleven people suspected of involvement with the well-planned assault were in custody but he did not say how many, if any, were gunmen taken alive and how many may have been people arrested elsewhere.
A British citizen of Somali origin was detained at Nairobi airport, a Kenyan security source said. A British newspaper said he was a 35-year-old, trying to leave on Turkish Airlines.
It was unclear whether intelligence reports of American or British gunmen would be confirmed. Al Shabaab denied that any women took part, after British sources said the fugitive widow of one of the 2005 London suicide bombers might have some role.
Smoke still rose into the damp air on Wednesday morning above the Israeli-built mall that had been a symbol of Africa's economic rise that has drawn in foreign investors.
Faster growth has also created wider wealth gaps, adding to grievances tapped by several violent Islamist groups from Mali to Algeria and Nigeria to Kenya. All have espoused an anti-Western, anti-Christian creed.
U.S. President Barack Obama, whose father was Kenyan, said he believed the country - scene of one of al Qaeda's first big attacks, in 1998, when a bomb devastated the U.S. embassy in Nairobi - would continue to be a regional pillar of stability.
Al Shabaab, which taunted Kenya when militants were battling inside the mall, said action by Kenyan troops using gas were responsible for the "lives of the 137 hostages who were being held by the mujahideen (fighters)."
"After 4 days of exposing the vulnerability of their nation, the Kenyan govt ended the siege in a morally reprehensible manner #Westgate," the group said on its Twitter account @HSM_PR
Kenyatta said he could not confirm intelligence reports of British and American militants. One minister denied speculation that women were among the guerrillas, but said some had been dressed as women, a possible ploy to get weapons past the unarmed private security guards who normally checked entrances.
It is unusual, if not unknown, for Islamist militants to use female fighters: "We have an adequate number of young men who are fully committed & we do not employ our sisters in such military operations #Westgate," al Shabaab said on Twitter.
The group dismissed comments by one Kenyan minister that two or three of the militants were young Somali or Arab Americans.
A British security source said it was possible Samantha Lewthwaite, widow of one of the London suicide bombers of July 7 2005, was involved in the Nairobi siege. "It is a possibility. But nothing definitive or conclusive yet," the source said.
Lewthwaite is wanted in connection with an alleged plot to attack expensive hotels and restaurants in Kenya.
Kenyatta thanked other leaders, including Obama, for their support and used his address to praise the response of the Kenyan people and call for national unity, six months after his election was marked by ethnic tensions.
"Kenya has stared down evil and triumphed," he said.
Many Kenyans agree that the bloodshed has helped foster a greater sense of national unity.
"We are all talking about it. The one good thing is that the whole of Kenya has become one, except for al Shabaab," said Vipool Shah, who helped pull bodies out of the mall.
Kenyatta's focus on Kenya's troubles, and of his role in a global campaign against terrorism, was a reminder that he faces trial at The Hague in a few weeks time for crimes against humanity over violence that followed a 2007 election.
The International Criminal Court adjourned the trial of his vice president this week because of the Westgate attack.
Kenyatta and his government have urged the ICC to drop the case and warm words for the Kenyan leadership from Western allies during the siege may have boosted their hopes that the court might be pressed to shelve proceedings in the interests of shoring up an important partner in the fight against al Qaeda.
Al Shabaab had threatened revenge since Kenyan troops joined the war against Islamists in its northern neighbour two years ago. The group created funding, recruiting and training networks in Kenya.