An investigative report by Finland's national broadcaster Yle has found alleged links between a Daesh* terrorist and a Finnish Islamic leader, who denies any radicalisation among his flock, yet promises that any returnees will be "treated nicely" and rehabilitated.

A man who left Finland to join Daesh was the son-in-law and business partner of the imam of the Islamic Society of Northern Finland, headquartered in the city of Oulu, Yle reported.

As part of its investigation, Yle found that Finnish jihadists earned money through a variety of legal and illegal means ranging from running pizzerias to tax fraud.

One of them, Bangladeshi-born Taz Rahman, was found to have co-owned a business with Oulu imam and fellow Bangladeshi Abdul Mannan. According to leaked Daesh documents, Rahman joined the terrorists' ranks in the summer of 2014. His wife and the imam's daughter, with whom he had children, followed suit.

Before his departure, Rahman ran two companies in Finland. One was a pizzeria that still operates in Helsinki. Rahman's second business was registered as a food kiosk at the address of the Oulu mosque in the city centre.

Daesh announced Rahman's death and in May 2017 published a photo of him, adding that he had worked as physical therapist; Mannan still runs the Oulu Islamic Society.

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When confronted by Yle journalists, Mannan denied several times personally knowing anyone who had been radicalised, despite the fact that his own daughter and son-in-law had become jihadists.

"I know each and every person. We don't have any extremists here," Mannan assured. When asked about Rahman, he admitted "he's a relative of mine, my daughter's husband, that's all."

Mannan also claimed that he didn't know where the couple was heading before they left. Mannan said he does not know whether his daughter is still in Syria, and claimed to have not heard from her in nearly two years and denied any responsibility.

"It's the person, the individual case, who is responsible for this. So if someone takes some decision for his own and without consulting us, so he's solely responsible for that," he stressed.

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Mannan said he had informed the police about the couple and ventured that there may be other Finnish nationals who have made the same choice.

"All of them are members of this society," Mannan concluded. "So when they come back, they will be treated in a nice way. They will be rehabilitated and we will let them live a normal life. I hope it will work."

Mannan previously served as a deputy city councillor for the Social Democratic Party from 2012 to 2017. Mannan moved to Oulu from Bangladesh in 1992 to study geology and earned a doctoral degree from the University of Oulu in 2002 before becoming an Islamic instructor.

Oulu has 200,000 residents and a 3,000-strong Islamic community. Some 1,000 people from more than 20 countries are said to attend events at the mosque.

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*Daesh (ISIS/ISIL/IS/the Islamic State) is a terrorist group outlawed in Russia and numerous other countries