In their email dated August 12, to family members of Foley, the Islamic State or ISIS, claimed that the US has refused several offers of release of the journalist, which included not only money but also the exchange of prisoners.

"You were given many chances to negotiate the release of your people via cash transactions as other governments have accepted. We have also offered prisoner exchanges to free the Muslims currently in your detention like our sister Dr Afia Siddiqqi, however you proved very quickly to us that this is NOT what you are interested in," said the email.

"You have no motivation to deal with the Muslims except with the language of force, a language you were given in 'Arabic translation' when you attempted to occupy the land of Iraq!" the email said, adding Foley would be "executed".

The Islamic State this week released a video showing one of its members beheading Foley, who was abducted in Syria in November 2012.

Dr Siddiqui, an MIT-trained neuroscientist, was arrested in Afghanistan in 2008 and found to have documents on chemical weapons, dirty bombs and viruses indicating she was planning attacks against American enemies, reports said.

Later, she was jailed by a US court for 86 years. 

"Now you return to bomb the Muslims of Iraq once again, this time resorting to Aerial attacks and 'proxy armies', all the while cowardly shying away from a face-to-face confrontation! Today our swords are unsheathed towards you, government and citizens alike! and we will not stop until we quench our thirst for your blood," the email said.

US President Barack Obama has vowed to bring to justice those responsible for killing Foley. According to Global Post, the email was published after the Foley family agreed to release it.

"Global Post has chosen to publish it in full in the interest of transparency and to fully tell Jim's story. We believe the text offers insight into the motivations and tactics of the Islamic State," the news website said.

The Global Post said in addition to the extreme views it espouses, the email contains factual inaccuracies. For example, the Foley family was not "given many chances to negotiate" for his release.

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