Still image from undated video of a masked Islamic State militant holding a knife speaking next to man purported to be James Foley at an unknown location
A masked Islamic State militant holding a knife speaks next to man purported to be U.S. journalist James Foley at an unknown location in this still file image from an undated video posted on a social media website. Britain is close to identifying a suspected British national shown beheading American journalist James Foley in a video released by Islamic State militants last week, the British ambassador to the United States said on August 24, 2014. Reuters/Social Media Website

Peter Theo Curtis, an American journalist held captive in Syria for the past two years, has been freed on Sunday.

His family credited the Qatari ruling family for the negotiations that led to his successful release from his captors, identified as Jabhat al-Nusra, an al-Qaeda affiliate. They are not in any way connected to the Islamic State group that assassinated James Foley, also an American journalist.

"We were repeatedly told by representatives of the Qatari government that they were mediating for Theo's release on a humanitarian basis without the payment of money," Nancy Curtis, the journalist's mother, said in a statement.

His family, however, said they are not aware if ransom was made in exchange for Curtis' freedom.

Curtis was handed over to UN peacekeepers in the Golan Heights region. After a medical check-up, he was then transferred to US representatives.

Secretary of State John F. Kerry said the U.S. had been conducting discussions and negotiations "over the last two years" for Theo's release as well as the release of any Americans held hostage in Syria.

It remained uncertain if Curtis' release was influenced by Foley's death.

Read: James Foley: Release of Beheading Video by ISIS Meant to Attract More Recruits to Join the Cause - Intelligence

"It is our assessment that his release wasn't a direct response to Foley's murder," the Wall Street Journal quoted an unidentified senior U.S. administration official.

Just a week ago, the radical ISIS group released a video showing the execution of Foley. It likewise threatened to kill one more journalist in the same video, Steven Sotloff.

According to the U.S.-based Committee to Protect Journalists, at least 70 journalists have been killed and over 80 kidnapped during the three-year conflict in Syria.

Curtis was reported to own a PhD in comparative literature from the University of Massachusetts. He is fluent in Arabic and French.

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The Curtis family on Sunday appealed to the militants still holding captives to show mercy. "We appeal to the captors of the remaining hostages to release them in the same humanitarian spirit that prompted Theo's release," Curtis' mother Nancy said in a statement.

Curtis was handed over to United Nations peacekeepers in Syria's Golan Heights.

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