Former Guantanamo Bay detainee Moazzam Begg has claimed he was involved in negotiations to secure the release of British hostage Alan Henning before he was arrested.
Begg, who was released from prison last week after seven terror charges against him were dropped, approached the Foreign and Commonwealth Office in a bid to help secure Henning's release, human rights organisation CAGE said.
Islamic State released a video on Friday showing the murder of the British aid worker, and CAGE claims the government "squandered to an opportunity" to release Henning by arresting Begg in February.
"CAGE, represented by Moazzam Begg had been approached by the FCO and were actively involved in high levels negotiations to secure the release of Alan Henning up to a week before Moazzam Begg´s arrest," a statement said.
"One can only speculate that Alan may have been free if Moazzam had not been incarcerated for seven months and was able to continue his work in Syria.
"We cannot get Alan Henning back but we owe it to his family and friends to get to the truth of the government's role - or lack thereof - in securing his release and why Moazzam Begg, who perhaps could have secured his release, was arrested in the first place."
Colin Livesey says he "always thought his brother would be coming home" and his death means he no longer believes in hope.
Those that knew Alan Henning are in a state of shock at the brutal end to his aid mission to Syria.