The man who became the Islamic State militant known as "JihadiJohn" was a relatively hardworking student who showed no signs of being radicalized, his former school principal said Monday.

Jo Shuter, former head teacher at Quintin Kynaston Academy in London, told the BBC that Mohammed Emwazi was a "hardworking and aspirational young man" when she knew him as a teenager.

"He was quiet, he was reasonably hardworking," Shuter said. Emwazi had "adolescent issues" and was bullied at school, she added, but he eventually settled down and did well enough academically to be admitted to the university that was his first choice.

"I can't stress enough, he wasn't a huge concern to us," she said.

Emwazi was revealed last week to be the masked Islamic State militant known as "Jihadi John" who appears in several online beheading videos brandishing a large knife.

Authorities are working to understand how he became radicalized.

The Kuwait-born Emwazi came to Britain as a small child, attending state schools in London before studying computer science at the University of Westminster. He left for Syria in 2013.

He was interrogated by security services while in Britain but was never arrested or charged.

Shuter said neither Emwazi nor other students showed signs of embracing radical causes while at school.

Two other former students from the same school were also thought to have gone to fight in Syria and Somalia.