According to a report in der Standard newspaper the fair-haired teenager’s former classmates recognised him in the video. He was filmed in what appears to be a slaughterhouse in Syria, and sheep carcasses can be seen in the background.
The 16-year-old, named as Oliver N, is reported to have sent threatening and insulting text messages to some of his classmates just a few weeks ago. He has been officially listed as missing since August 23rd.
According to der Standard he had been studying at a vocational college for Industry, Economy and Transport in Meidling. It’s thought he may have first come into contact with Islam in May, when he was approached at a party and persuaded to visit a mosque.
"He was a very nice student, but towards the end of the last school year he converted to Islam and at this point his behaviour changed," the school’s director Erich Macho told the Austrian state broadcaster ORF.
“He was very open about his conversion - and it was also obvious by his behaviour and things that he said,” Macho added. “In hindsight it’s easy to say that we should have done something - he was also distributing Islamic material to students and teachers.”
“It was as if someone had turned on a switch in him,” a friend, Claudia, told der Standard. She added that he had posted a picture of a veiled woman on Facebook and referred to her as his wife.
She said that she hoped he would come back to Austria. "He was a very nice person who hadn't had an easy life."
Media reports said that the teenager was born in Linz but had been living in Vienna for some time.
The Interior Ministry believes that up to 160 Austrians are currently in Syria, fighting with Isis militants.
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Recent reports suggest that two teenage girls who left Vienna to fight in Syria but now want to come back home could face up to five years in jail on their return.
The government has made it clear that participation in a terrorist organization - such as Isis - will be punished if fighters or their supporters return to Europe.