An investigation by German authorities into the circumstances surrounding the death of a young Jewish man involved with a “political cult” has been wound up.

Prosecutors in Wiebaden looking into the death and subsequent police investigation of Jeremiah Duggan, whose body was found by a road in 2003, have closed the long-running inquiry, to the dismay of the young man’s mother.

Erica Duggan campaigned for years for a thorough investigation and hopes were raised in 2012 when the Frankfurt High Court found mistakes had been made.

Jeremiah, a 22-year old Jewish student involved with the Wiesbaden-based group Helga Zepp LaRouche when he died, called his mother saying he was in danger and asking to be picked up shortly before he was found dead.

Erica, who lives in Golders Green, described LaRouche as “a cult” and said German authorities had not followed through with their investigations, saying: “Vital lines of enquiry have been ignored.”

German police recorded Jeremiah’s death as suicide, his body having been found on the autobahn near Wiesbaden, hours after attending an event organised by LaRouche, which he believed was a conference concerning the problems in Iraq.

A three-day inquest in the UK in 2015 heard evidence of possible “foul play” after Jeremy Hyam, the Duggan family’s lawyer, said a court in Germany “concluded that the facts as found by the prosecutor appeared to be impossible in the light of the evidence available”.

This week the family said its lawyers would be discussing “how vital witnesses have still not been investigated, suspicions and contradictions not followed up, and how the mindset of suicide has continued to blind the authorities”.