Jeremiah Duggan, 22, was hit by a series of vehicles on a road outside Wiesbaden, western Germany, in March. German police concluded that Mr Duggan, who was studying English literature at the Sorbonne in Paris, had committed suicide.

But following private inquiries, his family are pressing for the German authorities to take a fresh look at the case and at the activities of the group he was staying with. The Metropolitan police yesterday confirmed its officers are liaising with their German counterparts and Interpol.

He had travelled to Wiesbaden with a group of young men selling the newspaper Nouvelle Solidarité, a French version of a newspaper published by Lyndon LaRouche, an American rightwing extremist condemned by leading Jewish organisations as an anti-semite. LaRouche served five years of a 15-year sentence for fraud and conspiracy in 1984, has a following in many countries and has contested seven presidential elections as a minor candidate. The Schiller Institute in Wiesbaden, where the conference was held from March 21 to 23, is run by his German wife, Helga.

Mr Duggan's relatives complain the German investigation was insufficient. Erica Duggan, 57, a retired teacher from north London, said: "We have told them from the start that there is no way this was suicide." Her solicitor, Jackie Turner, said: "There are too many unknowns. Too many things were not followed up."

Mr Duggan, a talented poet, arrived in Paris in September 2001 and was studying both at the Sorbonne and the British Institute, where he was doing a degree in French. He had never been politically active, but as the war with Iraq began he decided to join the protests.

In March, he met a man selling newspapers outside the Hotel des Invalides, close to the British Institute. Mrs Duggan recalls: "He was very impressed with him and 'his wonderful ideas'." Struck by references to LaRouche, Jeremiah asked his mother to look him up on the internet, but it was not considered a priority.

Conspiracy theories

Had they dug deeper they would have been concerned. The Guardian has detailed some of LaRouche's "conspiracy theories". The former Trotskyist has spoken of a worldwide Zionist conspiracy taking in the freemasons, Henry Kissinger and the royal family. He described senior members of the Bush administration as "Children of Satan".

LaRouche made the war in Iraq a touchstone issue and it was the broad anti-war stance that attracted Jeremiah. "He said he had been watching pictures of the war and it was terrible. He was determined to join a protest," explained Mrs Duggan. "He told me and his French girlfriend Maya that he would be travelling to Germany with his new friends. She told me later that she had a bad feeling about them."

On March 22, Jeremiah phoned his father to wish him happy birthday and told them he was at the Schiller Institute. Lyndon LaRouche spoke at the conference and Jeremiah apparently asked him questions. On the Monday night, he told his girlfriend the lectures had been interesting.

But after three more days without contact, Mrs Duggan began to worry. "I woke up at 2.30am on Thursday and I didn't know why. I just sat next to the phone and at 5.24am it rang. He was terrified. He said, 'Mum, I am in deep trouble.' I asked him what the problem was and he said, 'You know Nouvelle Solidarité. I want out. I don't want any more.'" They were cut off, but Jeremiah called back moments later. "I could tell he was in terrible distress. He said, 'I want to see you, now!' and that he was in Wiesbaden. Then the line cut." Mrs Duggan called Colindale police station and then Maya, who had also received a disturbing phone call. "He told her he had found out some very grave things and that he was going to get the train the next day. He complained of pains in his arms and legs. Later that morning, his girlfriend also received a call from someone called Sebastian who said Jeremiah had run off."

At 3.30pm, two Met officers arrived to tell Mrs Duggan that her son was dead. "They said it was suicide. I shouted and screamed that there was no way."

Mrs Duggan and her husband, Hugo, travelled to Wiesbaden the next day, but felt the case was already closed. "They said he had psychological problems, because that is what Sebastian told them. They said he ran five kilometres out of Wiesbaden and that at 6am he had run in front of a car and knocked off the wing mirror. The police said he kept running and was hit by another car 10 minutes later and was killed. Another car then ran over his body."

But as the Duggans pieced together details of the conference, one anecdote disturbed them most of all. "It seems he had no idea about who he was with at all. Someone blamed the Jews for the war in Iraq and for the problems of the world. Jeremiah stood up and said 'But I am a Jew'. Everyone went quiet. Jeremiah would have been very upset by that."

An inquest has been opened and adjourned at Hornsey coroners court, in north London.

The Duggans know how he died, but remain determined to find out why. The Foreign Office has accepted the result of the German investigation but says it will help the family raise concerns.