A British woman is thought to be among militants who killed at least 67 people at a Kenyan shopping centre, the country's foreign minister has said.
Amina Mohamed said the woman had "done this many times before" and "two or three" Americans were also attackers.
Earlier, Kenya's interior minister said all of the militants were men although some may have been dressed as women.
Six British nationals are confirmed dead in the attack on the Westgate centre in Nairobi.
The Foreign Office (FCO) said the next of kin of the six had been informed and were being offered consular support.
It also said it was aware of the comments by Kenya's foreign minister, but officials refused to speculate on the identity of any of the perpetrators.
In a televised address to the nation, Kenyan President Uhuru Kenyatta said the four-day siege was over and declared three days of national mourning starting on Wednesday.
Mr Kenyatta said intelligence reports suggested a British woman and "two or three" Americans citizens may have been involved in the attack.
He added: "We cannot confirm the details at present but forensic experts are working to ascertain the nationalities of the terrorists."
He confirmed 61 civilians and six soldiers had died, and five attackers were shot dead by troops and 11 suspects were in custody.
At a press conference in Pakistan, Home Secretary Theresa May said: "I'm aware that there have been reports of a British woman being involved, but until we have seen the investigations completed it is not possible to give further details to confirm or deny that issue."
Responsibility for the attack has been claimed by al-Shabab - an Islamist group based in neighbouring Somalia which is part of the al-Qaeda network.
Ms Mohamed told the PBS NewsHour programme on Monday: "From the information that we have, two or three Americans [were involved] and I think, so far, I have heard of one Brit... a woman... and I think she has done this many times before."
The Americans are believed to have been aged about 18 or 19, of Somali or Arab origin and lived in "Minnesota and one other place", she said.
The comments have fuelled speculation about the possible involvement in the attack of British terror suspect Samantha Lewthwaite - the widow of 7 July suicide bomber Germaine Lindsay.
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.
The Metropolitan Police said a team of officers had travelled to Nairobi to help with "post-incident procedures" including gathering scientific evidence and supporting the work of British coroners.
A UK man, who lost his young daughter, told the Daily Telegraph she was killed by "animals" who were using "religion as an excuse".
Louis Bawa, of Leamington Spa, Warwickshire, said he had spoken to his daughter Jennah, eight, and Kenyan-born wife Zahira on Friday but "didn't get a chance to catch up with them" again.
He said they had gone on a regular Saturday shopping trip to the Westgate centre, but added: "This time they didn't come home."
Architect Ross Langdon - who had British-Australian citizenship - has also been named as one of the UK victims, although the Foreign Office is still not confirming identities.
Mr Langdon's company, Regional Associates, issued a tribute to him and said he had died alongside his pregnant partner, Elif Yavuz, a Dutch citizen.
British woman Lynsey Khatau, 23, from south Wales, managed to flee from the attack with her young son and husband.
She said: "It was really horrific. My son is really traumatised. We were going to die, that was the only thing I was thinking, we're going to die. There were people just falling everywhere."
A Leicester man brought up in Kenya told BBC Radio Leicester that four of his family members had been killed in the attack.
He said that he had spoken to his brother in Nairobi but "contact is difficult because they're in a balance between trying to find out if other people are still inside because we don't know where they are. It's just a really bad time."
According to the Sun newspaper, a four-year-old British boy was among the survivors of the atrocity.
His mother was reportedly shot in the leg, but was able to escape with her son and his six-year-old sister after the boy told one of the gunmen: "You're a very bad man."
The Daily Mail reported that an off-duty British Special Air Services (SAS) soldier who was in the shopping centre at the time of the attack saved at least 100 lives.
The newspaper reported that the unnamed man went in and out of the Westgate centre a dozen times to rescue people.
British nationals concerned about friends or family can contact the Foreign Office on +44 (0)20 7008 0000.