Authorities in Austria are facing questions over how a known extremist previously jailed for attempting to join Islamic State was free to carry out a terror attack in Vienna.

Isil on Tuesday claimed responsibility for Monday's attack in central Vienna in which four people were killed and 22 injured. 

The lone gunman who opened fire in streets crowded with revellers was named by Austrian authorities as Kujtim Fejzulai, a 20-year-old Austrian citizen of Albanian heritage.

Fejzulai was jailed last year after he travelled to Turkey in an attempt to reach Syria and join Isil, but he was freed last December after convincing Austrian authorities he had been deradicalised. 

On Monday he posted an image of himself brandishing a Kalashnikov, a handgun and a machete on his Instagram page and pledged allegiance to Isil. 

At just after 8pm local time (7pm GMT), wearing a fake suicide vest, he opened fire in the street outside Vienna’s main synagogue. Footage from the synagogue’s security camera shows him gun down a young man standing outside the Kaktus Bar before returning to shoot him again with the handgun.

In the eight minutes that followed, he made his way through Vienna’s “Bermuda Triangle”, a popular nightlife area packed with bars.

It was an unusually warm night and the outdoor tables were packed with people enjoying a last night out before a new coronavirus lockdown came into effect.

Fejzulai opened fire on them indiscriminately, criss-crossing his way through the alleys. He exchanged fire with two police officers, injuring one of them seriously, and left another man in a pool of his own blood outside the Salzgries restaurant.

He began to radicalise in his early teens, and was captured in an Isil safe house in Turkey in 2018 attempting to cross the border to Syria.

He had been monitored by Austrian intelligence for some time, as he had announced his intentions on social media.

At his trial he testified that he had got involved with “the wrong mosque” in 2016. By 2018, he was facing problems at home and at school and made plans with a friend to travel to Afghanistan and join Isil forces there.

He was jailed for 22 months but released last December after serving only seven and a half.

“At the time I thought he was just a silly boy,” Nikolaus Rast, his defence lawyer, told the Telegraph. “We all made mistakes. The court did and I did too. This is unforgivable.”