(Adds details from Islamic State email, paragraphs 8-11)
By Warren Strobel and John Irish
WASHINGTON/PARIS, Aug 21 (Reuters) - After months of silence from the captors of American journalist James Foley, on the night of Aug. 13, his family received a chilling message: Foley would be executed in retaliation for U.S. airstrikes on the militant group Islamic State.
The family passed the message on to the U.S. government. The FBI, which handles cases involving kidnapped American citizens, helped craft a response, pleading for mercy, said Phil Balboni, chief executive of GlobalPost, the Boston-based online news publication that employed Foley.
"It was an appeal for mercy. It was a statement that Jim was an innocent journalist" who respected the people of Syria, where he was held, Balboni said in a telephone interview.
Foley's family and friends hoped the militants were bluffing and wanted a ransom, he said.
On Tuesday, Islamic State militants stunned America with a gruesome video posted on YouTube showing the beheading of Foley, 40, by a masked, black-clad man who also threatened to kill a second American journalist, Steven Sotloff.
Foley's death, highlighting how Syria has become perhaps the most dangerous country in the world for journalists, followed intense efforts by GlobalPost and others to identify his captors, and despite brief email exchanges between the militants and his family in late 2013 about a possible ransom.
The captors demanded a ransom of 100 million euros, or about $135 million, for his release, according to a GlobalPost spokesman.
The Foleys did not hear back from the militants until this month's threatening email, which GlobalPost published in full on Thursday after it said the Foleys agreed to its release.
Addressed as "a message to the American government and their sheep like citizens," the email said: "You were given many chances to negotiate the release of your people via cash transactions as other governments have accepted."
The group added it had also offered prisoner exchanges for "Muslims currently in your detention."
Saying that the United States responded with airstrikes, the email ended with a declaration that Foley "will be executed as a DIRECT result of your transgressions towards us!"
The White House declined to comment on the warning about Foley but it said special operations troops were sent to Syria earlier this summer on a secret mission to rescue American hostages, including Foley, but did not find them.
"Since his capture, we have been using every tool at our disposal to try to bring him home to his family and to gather any and all information we could get about his whereabouts, his condition and the threats he faced," White House spokeswoman Caitlin Hayden said.
Obama vowed on Wednesday the United States would keep supporting Iraqis in the fight against Islamic State.
"AN EXTRAORDINARY PERSON"
Foley, who had previously been detained in Libya, was abducted on Nov. 22, 2012 - on the U.S. Thanksgiving holiday - near the city of Binnish in Syria's Idlib province, as he and his colleagues made their way toward the Turkish border.
Who initially seized Foley has been a subject of dispute. Some signs pointed to the Shabiha, militias loyal to the government of Syrian President Bashar al-Assad.
Balboni said there had been strong indications that Foley had been transferred to the Syrian capital Damascus. That information later proved incorrect.
The first solid information about Foley's condition, he said, came nearly a year after his abduction, from a returning European jihadist, or Islamic fighter, who had been with the American journalist in the city of Aleppo. This person provided confirmation that Foley was alive, as well as first-hand details of his captivity and his captors.
Foley was moved a number of times, and passed through the hands of various captors, Balboni said.
Didier Francois, a veteran French war correspondent who was held with Foley and released with three other French hostages in April, said he had little doubt Foley was under the control of Islamic State or its affiliates the entire time.
"The guy who killed him is the guy who took him from the start," Francois told Reuters.
Francois said he had been held with Foley from last August until April and that he was also held almost nine months with Sotloff.
"He was an extraordinary person with a strong character. He was a pleasant companion in detention because he was solid and collective. He never gave in to the pressure and violence of the kidnappers," Francois said of Foley.
Francois, who said he shared a cell with Foley beginning in October, said he had not spoken about Sotloff or Foley until now because the kidnappers had threatened to kill the remaining hostages if they did.
Another released Frenchmen, Nicolas Henin, told France's Express magazine that Foley had been treated worse than the other captives, after militants searched his computer and discovered his brother was in the U.S. Air Force.
"Because of that and as he was American he got extra bad treatment. He became the whipping boy of the jailers, but he remained implacable," Henin told the magazine.
In November 2013, Foley's family received its first e-mail message from the journalist's captors, demanding a ransom and offering proof he was alive, Balboni said.
That exchange did not last long. "Very few" messages were passed, he said. "They were not loquacious," Balboni said of the captors. "They made their demands."
The communications channel soon went silent, and until last Wednesday, there were no further messages to the family.
The U.S. government says it has a firm policy of not paying ransom in kidnapping cases, or encouraging third parties to do so, a policy that differs from many European governments. The British government has a similar approach to that of the United States.
At the time, Islamic State was "busy, busy releasing and ransoming other hostages," Balboni said. "We believed that the American and British captives were always going to be held for last."
Foley was one of dozens of journalists abducted in Syria during its 3-1/2-year civil war.
Not all of their names have been made public at the request of their families or news organizations that employ them. They include Sotloff and Austin Tice, who disappeared near Damascus in August 2012. Nothing has been heard of Tice since a brief video uploaded to the Internet in September 2012.
The New York-based Committee to Protect Journalists said it has documented 80 journalists who have been abducted in Syria since 2011, including 65 in the last year alone. Many of them are native Syrians, said CPJ's deputy director Robert Mahoney.
"We have never documented so many kidnappings in a single conflict as we have in Syria," Mahoney said.
About two dozen journalists are still believed held captive in Syria, with several others missing.
Until Foley's killing, militants had kept most foreign hostages alive in hopes of securing a ransom or political gain, Mahoney said. (Additional reporting by Steve Holland and Peter Cooney; Editing by Jason Szep and Ken Wills)