Claims by the Israeli authorities that a suicide bomber and his accomplice who attacked a cafe in Tel Aviv were British have rocked the UK's Muslim community.
Authorities have not confirmed the passports are genuine
A man carrying a passport in the name of Asif Mohammed Hanif, 21, killed himself and three other people when he detonated explosives on Tuesday.
An accomplice, thought to be Omar Khan Sharif, 27, is also said to be a British citizen, from Derby.
He fled the scene after his explosive belt failed to detonate.
It is the first time British citizens have been linked to an attack of this kind.
Mr Hanif's family spoke of their disbelief that he could have been involved.
His brother Taz, speaking from his home in Hounslow, west London, said his brother was studying Arabic at Damascus University in Syria so that he could return to England and teach children.
"He wasn't that sort of person," he said.
"Anyone who knew him would tell you. He was just a big teddy bear - that's what people said about him."
He told the Sun newspaper: "We used to watch the news and our parents said the suicide stuff is not good.
"What do you achieve by killing yourself and killing other people?"
People are very eager to support their Muslim brothers and sisters in Palestine and in other areas as much as they can
The BBC's Aeneas Rotsos said the Muslim community in Derby were genuinely shocked and surprised by the claims that someone from their community was involved in a suicide bombing in Israel.
"They say they are completely unaware of any members of their community being in Israel at this time," he said.
"They also say that the names Sharif and Hanif are very common among the Pakistani community."
He said the community was keen to point out that during the conflict in Afghanistan there were rumours that Afghanistanis from Derby were involved.
But those rumours came to nothing.
Muslims at the Derby Islamic Centre and mosque said they had not heard of an Omar Khan Sharif.
One said: "It is a big shock and we all get a bad name at the end of the day.
Police are guarding the former home of Mr Sharif in Derby
"All Muslims aren't terrorists."
Another said killing innocent people was not right, whether they thought it was part of their Muslim belief or not.
"But then again if you are suffering you will do anything," he said.
The attacker who is said to have died, Asif Mohammed Hanif, came from Hounslow in west London.
A spokesman for the Muslim Council of Britain, Inayat Bunglawala, said he did not know Mr Hanif but knew people who had seen him in London.
He said: "I have spoken to people who have seen Hanif handing out leaflets urging Muslims to donate to the Palestinian cause outside a mosque in Hounslow.
"This is a very tragic situation indeed."
The Foreign Office has confirmed it is working with the Israeli authorities, but could not yet confirm that the passports were actually genuine or that the suspects were really British.
Israeli authorities say Omar Khan Sharif is now high on their wanted list.
UK spokesman for the minority Muslim group Al-Muhajiroun, Anjum Choudhury, said it came as no surprise that young men from Britain may have been involved.
"It has not come as a surprise at all," he told BBC Radio 4's Today programme.
"The feeling for Jihad (holy war) at the current time in the light of Iraq and Afghanistan and the continuing intifada in Palestine is very hot within the Muslim community.
"People are very eager to support their Muslim brothers and sisters in Palestine and in other areas as much as they can.
"Of course, the ultimate sacrifice for a Muslim is to lay down his life for the sake of Allah in the struggle of Jihad."
England's first Muslim MP, Khalid Mahmood, called for action to stop extremists inciting people to carry out such attacks.
The Labour MP for Perry Barr, Birmingham, said it was devastating news at a time when there were efforts to launch the "road map" for peace in the Middle East.
"By these single actions they put on hold for the whole of the nation peaceful progress for the Palestinians and the Israelis," he said.