French magistrates have opened an inquiry into “involuntary manslaughter” after a man trying to cross into France from Italy died of hypothermia.

A lorry driver found Derman Tamimou on Thursday morning unconscious on the side of a highway that links Hautes-Alpes with the northern Italian region of Piedmont. Tamimou, 29, from Togo, was taken to hospital in Briançon, but it is unclear whether he died there or was already dead at the scene.

“The second hypothesis is the most likely,” said Paolo Narcisi, president of the charity Rainbow for Africa. “He was probably among a group of 21 who left the evening before, despite all the warnings given to them by us and Red Cross volunteers about how dangerous the crossing is.”

Tamimou was found between Briançon and Montgenèvre, an Alpine village about 6 miles from the border.

Italy and France’s refugee dispute awakens a dark colonial legacy Read more

Narcisi said his charity was working with colleagues in France to try and establish whether the rest of the group arrived safely. He said they most likely took a train to Oulx, one stop before the town of Bardonecchia, before travelling by bus to Claviere, the last Italian town before the border. From there, they began the mountain crossing into France.

“Every night is the same … we warn people not to go as it’s very dangerous, especially in winter, the snow is high and it’s extremely cold,” Narcisi said.

Tamimou is the first person known to have died while attempting the journey this winter. Three people died last year as they tried to reach France via the Col de l’Échelle mountain pass.

The movement of people across the border has been causing conflict between Italy and France since early 2011.

Matteo Salvini, the Italian interior minister, on Thursday accused France of sending more than 60,000 people, including women and children, back to Italy. He also accused French border police of holding up Italian trains with lengthy onboard immigration checks.

Last year, seven Italian charities accused French border police of falsifying the birth dates of children travelling alone in an attempt to pass them off as adults and return them to Italy.

While it is illegal to send back minors, France is not breaking the law by returning people whose first EU landing point was Italy.

“Some of the returns are illegal, such as children or people who hold Italian permits,” said Narcisi. “But there are also those who are legally sent back due to the Dublin agreement. So there is little to protest about – we need to work to change the Dublin agreement instead of arguing.”