A family of seven feared to be heading for Syria bought a one-way ticket to Turkey, a senior police officer said.
Urgent appeals have been made for information about Imran Ameen, 39, his wife, Farzana Ameen, 40, and their five children, aged between five and 15, from Bradford.
Community leaders have said they are extremely concerned the family may be heading for Syria, especially with such young children.
Those fears were echoed by West Yorkshire Police Assistant Chief Constable Russ Foster who told the BBC: "The indication is that they have got a one-way ticket.
"We know from previous experience that Turkey is a gateway to Syria and also Iraq."
Mr Foster described both countries as volatile, adding: "They are there with young children and our concern is safeguarding those young children."
The family was last seen on October 5 but were reported missing on Tuesday.
Arshid Siddique, a relative who lives on the same street, described Mr Ameen as a quiet and nice guy and said the close knit street had been left in shock.
He told BBC Radio Leeds: "The strange thing is they never saw anyone before they went, never said goodbye. There's my sister up the road who Farzana is close with, my aunty down the road. She didn't see anybody, we didn't hear nothing from her.
"Even this morning I've spoken to her brother in Pakistan, he was unaware and yesterday I told him this is what's happening on the street that his sister is missing and he didn't want to believe it, he didn't believe it."
He said she had messaged her brother telling him she had done what was best for the kids.
"Totally out of the blue to us, as I said the next door house there is where her mother was and she's bed ridden, so she was looking after her mother day and night, so you never expect the same person to suddenly jump ship and leave her mum," he said.
Earlier this year, sisters Khadija Dawood, 30, Sugra Dawood, 34, and Zohra Dawood, 33, also from Bradford, went missing after going on an Islamic pilgrimage to Saudi Arabia with their nine children.
It is believed they entered Syria to join the Islamic State group.
West Yorkshire Police said they were working with relatives who are still in the UK as well with the Turkish authorities to try and secure their safe return.
Ishtiaq Ahmed, of the Bradford Council for Mosques, said he did not know the family but appealed for the community to tell the police any relevant information.
Mr Ahmed told BBC Radio Leeds: "I think West Yorkshire Police are right in making an appeal to the community and if there is anyone in the community who has information about the whereabouts of this family it's important that they give that information to the police so the police can do whatever needs to be done to make sure the children are safe."
Asked if his organisation had done enough, following the disappearance of the Dawood family earlier this year, he said: "I think over the years and months we have worked with our membership through mosques and through our faith schools and through other community relations to basically emphasise to individuals and families that Syria not a safe place for anyone to travel.
"And, particularly, it's not a safe place for young people and therefore I think people would be taking a great risk if they were to flee and go to that part of the world under the current circumstances."
He said: "We need to know more more information regarding this family - about their whereabouts and what their motivation may be.
"But generally I think people are listening and our taking our message very seriously but we need to go on and we need to keep on pushing this message."