"Take me with you!" pleads a teenage Syrian migrant as the Swedish journalist was stunned but after wrestling with his conscience agrees.
It's the spring of 2014. A group of journalists are making a documentary for Swedish public television SVT about the reaction of Europe's nationalist parties to the migrant influx, filming the refugees' life on the road.
In Greece, Fredrik, his cameraman and his interpreter meet Abed (not his real name). The scrawny 15-year-old is travelling alone, his goal to make it to Sweden where he has a cousin.
"It took 10 to 15 minutes maybe for me to get that question into my head, and to understand what he was asking me and to make up my mind," recalls the 43-year-old journalist, interviewed in the southern Swedish town of Malmo where he faces trial.
"Everything became more clear when it came down to that very question: 'What decision will I be able to live with in the future for myself?'," he tells AFP.
His two colleagues also agree to help the teenager. In Greece, they cancel their flight to Sweden, and prepare to accompany the young boy on his journey through Europe.
"It wasn't about journalism anymore. It was about 'Who am I?' 'What do I stand for, and can I really look back knowing that I didn't help a boy who was about to risk his life?'"
"I'm confident that we are right, that it isn't a crime," he says.
The Malmo prosecution authority says the case is about the law, not morals.
"Legally, once they helped him to come, it's a case of human trafficking," prosecutor Kristina Amilon insists.
"If they had been paid, the crime would have been aggravated," she notes.
Since 2015 -- when the number of asylum applications in Sweden soared (from 80,000 in 2014 to 160,000 in 2005), requiring the country to halt its generous refugee policy -- the number of cases of people helping illegal immigrants come to Sweden has skyrocketed.
A total of 116 people were charged with human trafficking in 2016, twice as many as the previous year and almost eight times more than in 2014. Those convicted risk up to two years in prison.
Swedish justice authorities have no statistics on the sentences imposed on the guilty, but they have ranged from fines to a suspended sentence.
A 35-year-old Syrian father who came to Sweden as a refugee in 2014 was for example handed a 160-euro ($170) fine by a Malmo court for having picked up his wife and children in Copenhagen and brought them to Sweden after their journey through Europe.
Onnevall's documentary aired in January 2015, unleashing a wave of support for the journalist. But at the same time, it led to a police complaint being filed against him, marking the start of his legal troubles.
"A lot of people understood the position we were in," says Onnevall, who has the full support of his employer, SVT.
"We understand their choice to help the boy. They found themselves in a difficult and completely unexpected situation in which a child, who was in danger, asked them for help," SVT communications director Sabina Rasiwala says.
After the broadcast, the documentary was reported to the Swedish Broadcasting Commission but was cleared of any wrongdoing.
According to Reporters Without Borders, it is rare for journalists in Europe to find themselves in similar legal trouble.
On February 8, a French photojournalist, Ben Art Core, will go on trial in Nice for aiding illegal immigrants.
Meanwhile, Abed was reunited with his cousin in Sweden. He was quickly granted permanent residency status, and his parents, brother and sister were able to join him in Sweden. Family reunification rules have since been tightened.
Abed has stayed in touch with Onnevall.