Issam Abuanza was a doctor, working for Britain's National Health Service for seven years. A British citizen of Palestinian heritage, he worked in several hospitals in the United Kingdom, specializing in endocrinology, the study of hormones.

But two years ago, according to an investigation by the BBC, he left his prestigious job -- and wife and two children -- in Sheffield, England, to fight on the battlefields in Syria, becoming the first known British doctor to join the Islamic State.

British officials estimate more than 800 people with U.K. links have traveled to Syria or Iraq to join radical Islamist groups. The Washington Post recently unmasked the final member of the "Beatles," a group of four Britons who oversaw the killing and torture of Western hostages. The most prominent figure, Mohammed Emwazi, also known as "Jihadi John," made headlines around the world after videos were posted of his gruesome executions. He was killed last year in an airstrike.

But the fact that a British doctor was recruited to the cause -- someone who previously made his living caring for people -- has proved especially galling for some.

The BBC said that the revelation came from leaked ISIS recruitment papers. It cautioned that it hasn't been able to verify the authenticity of all the documents, but added that many have proved to be authentic.

Pictures taken from Abuanza's Facebook page, which was last updated in October, show the 37-year-old wearing blue hospital scrubs and a gun holster. In another, he is wearing combat fatigues and cradling a Kalashnikov rifle.

In a social media post from February 2015, he wrote about the Jordanian pilot who was burnt alive by the Islamic State, which later released a video of his painful death. Abuanza said he would have tried to keep the pilot alive longer to prolong his torture.

"I would've liked for them to burn him extremely slowly and I could treat him so we could torch him once more," he wrote.

He also applauded the 2015 terrorist attacks in Paris at the Charlie Hebdo magazine.

"Praise be to God for this terrorist act. God kill off their enemies, military and civilian, men and women, adult and children," he wrote.

Speaking to the BBC, his sister Najla said that her parents would never forgive her brother, whom she said was once "quite the dashing young man, very modern."

"They will never forgive him," she said. "My dad's wish was to see him before he dies. He has spent all his money on him and his education and this is what he does."