2 of the ISIS executioner 'Beatles' captured in Syria
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WATCH: Rich Sullivan's son Justin Sullivan, who was a teen when he reached out to ISIS recruiters online, is spending life in prison for terrorism.

Two notorious British ISIS members believed to be involved in the brutal torture and beheadings of Western hostages are being detained by U.S.-backed rebel forces in Syria, U.S. officials confirmed. The pair were part of a group of four ISIS members from the United Kingdom nicknamed "The Beatles" by their hostages because of their English accents and origins.

According to two U.S. officials, Alexanda Kotey and El Shafee Elsheikh were recently captured by the Syrian Democratic Forces and are being interrogated by U.S. interrogators.

Kotey and Elsheikh were the two remaining members of the ISIS "Beatles" quartet who remained on the battlefield.

The group's ringleader, Mohammed Emwazi, who was nicknamed "Jihadi John", was killed in a 2015 CIA drone strike.

A video appears to show the execution of Steven Sotloff, the second American killed by a self-professed member of the Islamist terror group ISIS.

Emwazi had the highest profile in the group and narrated the gruesome ISIS videos that showed the beheading deaths of American hostages James Foley, Steven Sotloff and Abdul-Rahman Kassig.

Aine Davis, another member of the foursome, was reportedly captured and is being held in Turkey.

The pair's capture was first reported by the New York Times.

Mohammed Emwazi, also known as "Jihadi John", appears unmasked in ISIS photos.
Kayla Mueller poses after speaking to a group in Prescott, Ariz., May 30, 2016.

Kotey and Elsheikh were captured in early January by the Syrian Democratic Forces during an operation in eastern Syria, according to a U.S. official.

They are believed to have acted as guards and interpreters involved in ISIS’ illegal captivity of Western hostages, said the official.

Among those hostages was American Kayla Mueller of Prescott, Arizona, who according to U.S. officials, is believed to have been killed during her captivity.

ABC News' 20/20 has previously reported that Mueller was being held hostage by "The Beatles" and that she once boldly corrected Jihadi John when he wrongly told other hostages that she had converted to Islam.

A senior U.S. official confirmed that part of the interrogation of Kotey and Elsheikh is aimed at finding the remains of Western hostages, particularly Americans, who were killed during their ISIS captivity.