A member of the family of 12 feared to have travelled to Syria was stopped by police the day before they left, it has been claimed.
A 21-year-old woman was spoken to at Heathrow Airport on April 9 but was not arrested, before officers searched a property linked to the group, according to the BBC.
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.
Police have launched an investigation after the group from Luton failed to return from a holiday in Bangladesh.
Mr Mannan's son Mohammed Akhtar Hussain, who reported the family missing, told Channel 4 News his sister Ms Khanom was initially prevented from flying when she was pulled aside at the airport.
"She didn't have any of her own luggage to check in because everyone's luggage was mixed in together," Mr Hussain told Channel 4 News.
"But the officers thought this was odd as she was supposed to just be going on holiday."
He is adamant his father would not have wanted to go to a war zone.
"I've been worried sick about my father, he's not in the best of health," he said.
The party initially travelled to Bangladesh from Heathrow via Istanbul on April 10, before flying back to the Turkish city on May 11.
They were due to come back to London three days later but failed to return and were reported missing by a relative on May 17.