A film highlighting the "shattering" impact experienced by the families of those who go to Syria is being released in a bid to dissuade others from going.
"Left Behind" - a police and Prevent initiative - features accounts from police contact officers who support the partners, parents and siblings of people who travel to conflicts abroad.
The six-minute film will be released online later.
UK authorities say at least 800 Britons have travelled to Syria or Iraq.
In the film, one contact officer says: "One particular case, the son had gone to Syria and the younger child... keeps on going to the bedroom trying to find his older brother.
"When he finds an empty bed... He obviously can't understand what's going on."
It is an "emotional" role, another officer says, and a third adds: "It's very difficult to explain to the mother that she will never get to see her child's coffin because her son has been killed on a field in Syria."
The film also features a message to anyone who might know someone looking to travel to a conflict zone.
"Speak to the police as soon as possible, the sooner the better. Do it before it's too late," an officer says.
The film will be launched at a community event in Birmingham by national counter-terrorism police and the Prevent Tragedies campaign, which aims to stop people being drawn into extremism.
Who are Britain's jihadists?
At least 800 people from the UK have travelled to support or fight for jihadist organisations in Syria and Iraq, say the British authorities. About half have since returned to the UK.
Deputy Assistant Commissioner Helen Ball, who is senior national co-ordinator for counter-terrorism policing, said: "This film shows that, at a time when officers are working to establish the facts, they are also seeing mothers, fathers, brothers and sisters coming to terms with the possible loss of a loved one.
"It is shattering for all concerned.
"I hope that when people hear the accounts of our officers, they can have conversations in their own homes and communities and consider what they can do to stop this ever happening to another family."
The Press Association news agency said figures showed that in the year to the end of May, 84 people believed to have travelled to Syria had been reported missing by their families, including 49 aged under 21.
The total number has fallen compared with the previous year, when there had been more than 100.