The U.S. backed secret negotiations with the spiritual leadership of the Islamic State group, conducted by two al Qaeda-linked clerics and a former Guantanamo detainee, in a bid to secure the release of American hostage Peter Kassig, who changed his name to Abudul-Rahman after converting to Islam, according to an investigation carried out by the Guardian

The negotiation effort was the work of Stanley Cohen, a controversial New York attorney, who among others has represented Osama Bin Laden's son-in-law, Abu Gaith, in U.S judicial proceedings.

According to the Guardian, Cohen enlisted the help of Abu Muhammed al-Maqdisi, a jihadi scholar, radical Islamist cleric Abu Qatada, and an unnamed Kuwaiti who was a veteran of al Qaeda and a former Guantanamo prisoner.

In return for Kassig's life, the clerics were offering to ensure that al Qaeda would cease its criticism of ISIS as being without proper Islamic credentials.

Senior FBI officials were aware of the effort, and had agreed to pay thousands of dollars in expenses incurred by Cohen, according to the paper.

The negotiations, which went on for a number of weeks, collapsed after Jordanian security services arrested Maqdisi, despite an agreement secured by Cohen that the country would not intervene in their attempts to contact ISIS.

Cohen told the Telegraph that here was now “no chance” that Islamic factions opposed to ISIS' hostage-taking would try to secure the release of other captives. 

Kassig was eventually murdered by his captors, and a graphic video was posted online last month purporting to show the aftermath of his killing. 

The negotiations are not the first incidence of al Qaeda intervening on behalf of a Western hostage held by ISIS. The group also appealed for the release of British hostage Alan Henning, because it believed he was an innocent aid worker who was genuinely trying to help suffering Muslims, according to the U.K.'s Independent

Al Qaeda has, in the past, also not shied away from expressing disquiet at the brutal tactics employed by other Islamist militant groups. In personal letters from Osama Bin Laden, described in Mark Bowden's book “The Finish: The Killing Of Osama Bin Laden,” the al Qaeda leader urged Abu Musab al-Zarqawi, then the leader of al Qaeda in Iraq, to refrain from killing fellow Muslims in bomb attacks.

The fate of many Western hostages, including Kassig, who have been killed by ISIS terrorists in recent months, has caused many in the U.S. and U.K. to re-examine the longstanding policy of refusing to negotiate with terrorists, or to pay ransoms for hostages.

The U.S. government is currently conducting a review of how it handles hostage situations, but it is thought that the policy is unlikely to change. 

In stark contrast to the U.S. and U.K., many central European nations have reportedly paid ransoms to secure the release of their nationals from the hands of groups like ISIS, attracting criticism that they are effectively bankrolling terrorist activity, according to the New York Times

In addition to the news story cited above, the Guardian has a long-form story on the secret negotiations for Kassig's life. You can read it here