Forensic experts are working to establish whether a British woman was among the Islamist militants who stormed a Kenyan shopping centre leaving more than 70 dead.
Kenyan President Uhuru Kenyatta said the al Qaida-linked terrorists who overran the Westgate centre in Nairobi on Saturday have now been "defeated" - with five militants killed and 11 others in custody.
He confirmed also that intelligence reports had been received suggesting that a British woman and "two or three" American nationals had been involved in the attack.
"We cannot confirm the details at present but forensic experts are working to ascertain the nationalities of the terrorists," he said in a televised address.
"We have ashamed and defeated our attackers."
Kenya says the terrorists who stormed a Nairobi shopping mall have been defeated, but three mall floors collapsed during the battle, 72 people are confirmed dead and more bodies are trapped.
Announcing three days of national mourning, Mr Kenyatta said that 61 civilians and six members of the security forces had been killed in the fighting, while 62 injured remained in hospital.
Towards the end of the operation, three floors of the complex collapsed and some bodies - including those of some of the terrorists - remained trapped in the rubble, he said.
"As I had promised earlier, we have ashamed and defeated our attackers. That part of our task has been completed by our multi-agency security team," he said.
"I promise that we shall have full accountability for the mindless destruction, deaths, pain, loss and suffering we have all undergone as a national family. These cowards will meet justice, as will their accomplices, wherever they are."
But while he said "the worst" of the crisis was now over, it was unclear whether Kenyan security forces had accounted for all the militants.
Earlier, al Shabaab, the Somali terrorist group which carried out the attack, claimed that its fighters were still resisting and holding a number of hostages.
Ministers have already confirmed that there are at least six British nationals among the dead, with fears the number could rise.
The confirmation that a British woman may have among the attackers fuelled speculation that it was the terror suspect Samantha Lewthwaite, who was married to the July 7 bomber Jermaine Lindsay.
Dubbed the "White Widow", she is known to be in East Africa and is wanted by Kenyan police over alleged links to a terrorist cell that planned to bomb the country's coast.
In March last year officials said she had fled to Somalia and that officers were hunting a woman who used several identities, including hers.
The Foreign Office earlier confirmed that it was aware of the claim, originally made by the Kenyan foreign minister Amina Mohamed while attending the United Nations General Assembly in New York.
"We continue to liaise very closely with the Kenyan authorities and to support their investigation into this attack," a spokesman said.
While Mr Kenyatta did not say whether the suspected British woman was among those in custody, the fact forensic tests are being carried out would suggest she was one of those who had been killed.
Earlier, in a statement to the Associated Press news agency, al Shabaab, had dismissed suggestions that a woman had been involved in the attack.
"We have an adequate number of young men who fully committed and ready to sacrifice their lives for the sake of Allah and for the sake of their religion, so there is no need for us to employ our sisters in the battlefield and thereby expose them to unnecessary risk," it said.
The latest disclosures will fuel concerns among the security services in Britain that Somalia is becoming a magnet for UK-based extremists.
The then head of MI5, Jonathan Evans, warned three years ago that he feared it was only a matter of time before jihadis fighting alongside al Shabaab were involved in acts of terrorism on the streets of Britain.
But while analysts will be looking for any evidence of an increased threat to the UK in the wake of events in Nairobi, the focus of British militants has switched overwhelmingly to Syria.