A family of 12 including two grandparents with health problems and a baby are feared to have travelled from England to Syria.
Police have launched an investigation after the group from Luton failed to return home from a holiday in Bangladesh.
Relatives said their disappearance does not make any sense, adding that they "can only think they have been tricked".
Muhammed Abdul Mannan, 75, and his wife Minera Khatun, 53, are missing, along with their 21-year-old daughter Rajia Khanom, 21 and sons Mohammed Zayd Hussain, 25, Mohammed Toufique Hussain, 19, Mohammed Abil Kashem Saker, 31, and Mohammed Saleh Hussain, 26.
Three unnamed children aged between one and 11 - a son of the older couple and two grandchildren - are with the group, as are Mohammed Abil Kashem Saker's wife Sheida Khanam, 27, and Mohammed Saleh Hussain's wife Roshanara Begum, 24,
The elder Mannan was described as "frail" and reportedly suffers from diabetes, while his wife is understood to have cancer.
In a statement issued through police, relatives of the group said they are "devastated" and "very concerned for their safety".
They said: "This is completely out of character and we are very worried of the danger they may now be in.
"This just does not make any sense. We can only think they have been tricked into going there, it is no place for elderly or young people.
The group were due to come back to London on May 14 but failed to return and were reported missing by a relative on May 17.
A police spokeswoman said: "There is a suggestion that the family may have gone to Syria, however police have so far been unable to corroborate that information.
Community leader Ashuk Ahmed said there were suggestions within the community that a woman in the group might have been "radicalised".
"The community is in shock. The boys were all running successful businesses," he said.
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