A senior administration official confirmed to ABC News that the White House knew of the threat in advance of the video’s release. The US intensified air strikes against Isil positions during the subsequent days.

Mr Balboni said that the family and Global Post had still remained hopeful even after receiving the email last Wednesday.

“But you can see the seething anger,” he said. “ It could have been a bluff and we had to believe it was a bluff. You know when you kidnap someone and hold them for almost two years, you don’t do it unless you believe there is value in those hostages.”