Maajid Nawaz was a teenager growing up in Essex when he first became involved with extreme Islamism, joining political organisation Hizb ut-Tahrir (HT), which seeks to establish an Islamic state across the Middle East.
For over a decade Nawaz was inside HT, not only propagating their views in Britain, but exporting them abroad.
Ten years ago, he travelled undercover to Pakistan as an agent of HT, infiltrating universities and working to win recruits.
The ultimate goal - to overthrow democratically elected governments and establish a worldwide government of Islam.
For being a HT member, he even spent four years imprisoned in Egypt, where the group is banned.
But in 2007, on an edition of Newsnight, Nawaz publicly renounced his beliefs and support for HT and took up a new role fighting extremism and spreading moderation amongst British Muslims.
In May 2009 he took that effort to Pakistan, in part to try to undo some of the damage that he himself did, and Newsnight joined him on his trip.
Nawaz was imprisoned in Egypt for four years for HT membership
In Pakistan, increasing numbers of children are abandoning secular schools in favour of madrassas - religious seminaries.
Pakistani police say the majority of jihadist suicide bombers come out of these schools, and there are concerns that the madrassas' narrow focus on sacred texts leaves the students ripe for radicalisation.
However, on his tour of educational institutions, Nawaz opted to focus not on madrassas, but on universities.
The majority of suicide bombers may come from poor rural areas and be madrassa-educated, but they are the foot soldiers, not the generals, he argues.
The leaders, the ideologues, he says, are university educated and it is there his focus should lie.
So for weeks, Nawaz criss-crossed Pakistan from Quetta to Karachi, from Lahore to Mirpur, giving speeches at universities.
Nawaz says he received a lot of support on his visits, but that he was also been on the receiving end of a lot of anger, mainly directed at the West:
Hizb ut-Tahrir members want to establish a Muslim Caliphate
When we joined him in Lahore one man in the audience said succinctly: "If America wants us to do something, then we won't do it."
The man also had a dig at Nawaz's mainly English speeches, peppered with Urdu:
"You come here as a Pakistani, I would first like to tell you to go and learn to speak the Pakistani language. Then you come here and tell us how to be good Pakistanis."
In Islamabad, a student from Peshawar in the North-West Frontier Province took issue with the idea that Pakistanis are responsible for the problem, saying:
"You said that we ourselves are responsible for this extremism and terrorism in Pakistan. But I don't agree with you. Because the leaders have come from outside. Before 9/11 there were no Taliban, terrorism. It started after 9/11. America, Israel and India are encouraging it."
Bullet proof vest
Nawaz said responses like this were common, but he has heard more from what he calls the "moderate majority".
"I've had people tell me that there's no extremism in Pakistan, but that contrasts with what the majority are saying," he explains.
And there has been real proof of that extremist threat.
One event had to be cancelled because of violence on campus. And ongoing death threats mean that when Nawaz does venture out in public he wears a bullet-proof vest.
Leading academic says the Taliban threat is a wake up call for Pakistan
But does Nawaz's experience paint a true picture of what is happening on Pakistan's secular campuses?
Quaid-I-Azam University in Islamabad is the country's most prestigious university.
Walking through its green and leafy campus, are four mosques, but its only bookshop closed down years ago.
For Dr Pervaiz Hoodbhoy, a well-known nuclear physicist and anti-extremism campaigner, who invited Nawaz to speak at the university, says that is a sign of the times.
Even 10 years ago he says, it would have been unusual to see a woman in a full veil, or niqab, on campus.
Now a woman who does not cover her head is the exception.
'Lack of curiosity'
For him though, that change in dress is only a symptom of a far more fundamental change:
"There is much less curiosity about the outside world something important, something fundamental has changed," he says. "You might ask what? I would say it is the education they are receiving in school, which has become so islamicised, so focused on so-called morality issues."
The sight of a niqab is no longer a rarity at Pakistan's universities
For Dr Hoodbhoy, inviting Nawaz to speak to his students is just one more way to try and open their minds. A task that he says is crucial to challenging the increasing power of the extremist ideology:
"The mullah, he appeals to something primal. He doesn't engage the mind. In school, they are taught to obey, obey, obey, that's what some of us in the university to break."
By that standard, it seems that Nawaz's tour is, at least in part, a success. At one of the lectures Newsnight attended, half the students raised their hand when asked if listening to Nawaz had changed their minds about extremism.
One young man told us that he had always thought extremism was just about poverty:
"He had every facility of life. He had everything. But he became a victim of extremism. That changed my view," he said.
And Nawaz says that every new recruit he gains this time can make a difference.
"My hope is to come here with others and together help to start a civil society and to reclaim Islam for both the UK and for Pakistan.
Viewers can watch the Newsnight film on Maajid Nawaz's Pakistan tour in full on Tuesday 23 June at 10.30pm on BBC Two, then on the Newsnight website.
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