London Terrorist Had Been Seen Unfurling ISIS Flag on British Documentary in 2016

The 27-year-old attacker, known only as Khuram Shazad Butt, was featured in "The Jihadis Next Door."

One of the terrorists behind the horrifying attack in London Saturday night once appeared in a British TV show about Islamic extremists that was called The Jihadis Next Door, it has been revealed.

“This black flag you see here one day is going to be on 10 Downing Street,” the 27-year-old, identified as Khuram Shazad Butt, says on the docuseries.

Read: Witnesses Describe Horror During London Terror Attacks: 'I Saw a Man Stabbing the Woman'

He is seen unfolding a black ISIS flag and praying with other radicals. When they are confronted by police, he puts on a pair of dark glasses.

Butt was a father of two who worked at KFC.

The TV documentary was broadcast in January 2016 and now British authorities are coming under heat how could he have slipped through the cracks.

On Saturday, he and two other terrorists killed seven people as they mowed down pedestrians on London Bridge before emerging from the van and stabbing people at random.

Butt and his terrorist cohorts were killed by police, who fired off more than 50 bullets in just several seconds.

Security expert Bob Strang spoke to Inside Edition about what happened. 

"Law enforcement has to make decisions about what is the most risk to the public,” he told Inside Edition. “They only have so much resource. Those decisions were made that there were other people who were far more dangerous than him."

Security in cities across America has been intensified as a result of the London attack, with an elite counter-terrorism unit deployed in Manhattan's Times Square.

London Bridge reopened Monday with sidewalks covered in flowers to honor those mowed down or knifed to death in the attack.

Londoners are taking offense at a headline in The New York Times that said they are “reeling” in the wake of the attacks.

Read: Manchester United: Royal Family, Celebrities Show Support for City After Attack

Harry Potter author J.K. Rowling posted: “Don't confuse grief with lack of courage."

HBO’s John Oliver spoke about Britain’s courage in the face of the attacks.

“British people are never going to let terror change their way of life and if you need proof of this look at this footage of people being evacuated from the scene,” he said on his show Last Week Tonight. “See that one guy there? He refused to leave his pint of beer behind!”

Inside Edition caught up with James Corden as he was preparing to broadcast The Late Late Show from London.

“It's unbelievably sad, it's tragic," Corden said. "It's a tragedy beyond belief and you sort of wonder how many times you have to turn on the television or radio and hear this news and hear these reports."

Watch: Canadian Social Worker Died in Fiance's Arms After London Terror Attack