The email, which is littered with grammatical errors, says the US government has refused to negotiate for the release of its captives through paying ransoms or releasing prisoners. But it directly blames American airstrikes for Foley's killing.
Warning: The text may be upsetting.
HOW LONG WILL THE SHEEP FOLLOW THE BLIND SHEPPARD?
A message to the American government and their sheep like citizens:
We have left you alone since your disgraceful defeat in Iraq. We did not interfere in your country or attack your citizens while they were safe in their homes despite our capability to do so!
As for the scum of your society who are held prisoner by us, THEY DARED TO ENTER THE LION’S DEN AND WHERE EATEN!
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 Sidiqqi, however you proved very quickly to us that this is NOT what you are interested in.
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!
Now you return to bomb the Muslims of Iraq once again, this time resorting to Arial 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 UNTILL WE QUENCH OUR THIRST FOR YOUR BLOOD.
You do not spare our weak, elderly, women or children so we will NOT spare yours!
You and your citizens will pay the price of your bombings!
The first of which being the blood of the American citizen, James Foley!
He will be executed as a DIRECT result of your transgressions towards us!
On Wednesday President Barack Obama's administration revealed that US special operations forces had attempted to rescue Foley and other American hostages in Syria earlier this summer, but had been thwarted.
The New York Times reported late on Thursday that Sotloff may be one of three American hostages ISIS has threatened to kill if its demands are not met.
US airstrikes against IS extremists in Iraq, to shore up Kurdish and Iraqi efforts to hold the strategically important Mosul Dam, have continued despite the threat. In the video of the murder of Foley, Obama was warned that the life of American hostage Sotloff hinged on his "next decision".
British spies are heavily involved in efforts to identify Foley's killer and his group of extremists, with the Cheltenham-based eavesdropping agency GCHQ playing a vital role in intercepting any electronic communications between jihadists in Syria and Iraq.
The British agencies - MI5, MI6 and GCHQ - were also working with American and European counterparts and receiving reports from agents on the ground to try and build a more complete picture of the scenario in Iraq and Syria.
However, much of the ongoing work will not deviate from everyday business as the hundreds of Britons who have travelled overseas to take up jihad have for some time been of central concern to the security services.
Foreign Secretary Philip Hammond said "significant numbers" of British nationals had been "involved in terrible crimes, probably in the commission of atrocities" with IS and other extremist groups.
Dr Andrew Mumford, a lecturer in politics and international relations at Nottingham University, said intelligence services faced a "big task" in trying to identify the murderer, and would be combing through people known to have links with Britons who have travelled to fight in Syria.
But he warned the agencies could face problems if the killer had travelled to the region "under the radar" with "no background in terrorism or any other criminal activities".
On the role of "The Beatles" in guarding kidnapped Westerners, he said: "A picture is emerging that IS is now explicitly using Western jihadists to take control and look after Western hostages. That is an important and interesting development."
A daring mission by US special forces this summer was launched in an attempt to rescue hostages including Mr Foley, senior officials in the Obama administration revealed.
The mission was authorised after intelligence agencies believed they had identified the location where the hostages were being held.
But several dozen special operations troops who were dropped by aircraft into Syria did not find them and engaged in a firefight with IS militants before departing.