New pictures emerge showing the happy teenager who, only around a year later, would become a militant Islamic extremist.

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The images, uncovered by Channel 4 News, show an apparently happy Aseel Muthana, whose older brother appeared in a propaganda video released by Isis, on a day out paintballing with friends. It is thought he was with friends from a mainstream Islamic society, which is not thought to have any links to extremism.

They emerge as an imam, who knew Aseel as an adolescent, describes the changes he and his brother Nasser went through as they transformed from "normal" teenagers to Islamist extremists.

Sheikh Zane Abdo, imam of the South Wales Islamic Centre, said Nasser Muthana, now 20, and his 17-year-old brother Aseel were "very well-spoken, very sincere", but went through a strange period in their lives.

He said the brothers "liked watching movies, did a lot of school" like normal teenagers. There was "nothing that suggested that they were going to go down the route that they went down", he said.

"However, they went through a very strange period when they said they were becoming quite serious in their faith and then began to start to expressing certain views that were quite political, particularly the older brother," he added.

"Neither of them in the past year and a half to two years frequented this mosque or attended any of the sermons or any of my classes that I have been giving for the past three and a half years," said Sheikh Abdo.

'Brainwashed'

Nasser Muthana and Reyaad Khan, 20, have been identified as two of the three British men in the Isis recruitment video, while the identity of the other remains unknown. Aseel Muthana is also believed to have travelled to Syria without his parents' knowledge. A fourth man appeared in the video, reported to be an Australian who has since been killed.

But a local community leader and friend of Khan's family said the young men could still be reintegrated if they returned home. Mohammed Sarul Islam told the Guardian newspaper: "This is a close-knit community that will be able to work together and bring them back to the good side from the bad side. I believe that can happen."

Reyaad Khan, 20, was said to have once dreamed of becoming Britain's first Asian prime minister. Nasser Muthana once planned to go to medical school, it has been reported. They are believed to be fighting with Isis, which has been involved in the war in Syria and has recently taken large swathes of neighbouring Iraq.

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Their families expressed their anguish after learning that their sons had travelled to join Isis. Khan's mother, who has not been identified, appealed to her only son to come home. She also said the young men had been "brainwashed".

"He is honest, always caring for his family, he always wanted to be there for them. He was one of the best boys a mother could ever want," she said.

"I think they are brainwashed into thinking they are going to help people. I don't know who it is but there is someone behind them, keeping these young, innocent boys, brainwashing them into thinking they are going to help people. There is someone behind them, I don't know who," she told Sky News.

It has emerged that Reyaad Khan grew up close to Abdul Miah, who was jailed along with eight other men in 2012 over a plot to bomb the London Stock Exchange.

Ahmed Muthana, father of Nasser and Aseel, also spoke of his devastation after seeing Nasser on the video.

'Betrayed'

The 57-year-old retired electrical engineer told the Guardian newspaper that he felt his son had betrayed his country. "This is my country. I came here aged 13 from Aden when I was orphaned. It is his country. He was born here in the hospital down the road. He has been educated here. He has betrayed Great Britain," said Ahmed Muthana.

He said that he felt as if a bomb had hit his Cardiff home when he saw the video featuring his son. "I was shocked, I was sad, I cried. My wife collapsed, it feels as if the ground under my feet has disappeared," he said.

They went through a very strange period when they said they were becoming quite serious in their faith and then began to start to expressing certain views. Sheikh Zane Abdo, imam

At the al-Manar centre, where the Muthana brothers and Khan were thought to have worshipped, trustee Barak Albayaty said: "Nasser Muthana was just like any other guy. I was shocked to see him in the video."

Speaking to the Guardian, he said: "But I am sure coming here is not the source of radicalism. We're against going to Syria for the armed struggle and have spelt this out on many occasions. The boys are affected by the internet. It's not just Cardiff, it's all over the UK."

Sheikh Abdul warned that the widespread publicity given to the Isis propaganda video in which the older Muthana brother was featured would encourage other "susceptible" young men to travel to Syria to fight.

He said a "platform" should not have been given to the recruitment video, which also features another Cardiff man Reyaad Khan, who went to school with Nasser Mathana.

"I guarantee that many young people who are very susceptible to this type of message will have watched that video and maybe have been encouraged to now go and follow in the footsteps of Nasser and his brother, which is a real problem, the fact that a platform has been given to this video that really shouldn't have been given," Sheikh Abdo told BBC Breakfast.

Security services estimate that around 500 British Muslims have travelled abroad to fight - most to Syria. They say around 300 have already returned to Britain.

His fears were echoed by Sir Peter Fahy, Greater Manchester Chief Constable and lead on the Prevent counter-terrorism strategy for the Association of Chief Police Officers. He said that the police simply do not know for sure how many British jihadis have travelled abroad.

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Richard Barrett, a former head of counter-terrorism at MI6, estimated that around 300 people have come back to the UK after fighting in Syria. But he admitted that it was just a "ballpark figure". He told Channel 4 News that it was only possible to follow a small number of those people closely because of a lack of resources.

Former defence secretary Liam Fox called for intelligence agencies to be allowed to gather more data on British citizens fighting abroad.

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