Eighteen months after the decomposing bodies of Petra Kelly and Gert Bastian were found in their house in Bonn, a few of the couple's friends still believe that some dark external agency played a part in their deaths. Most, however, accept - as the forensic evidence concludes - that Bastian murdered Kelly and then killed himself. For the police, the case is closed. It is not their job to look beyond the identity of the murderer. For the couple's friends, the mystery of Bastian's last act remains as cruelly tantalising as ever.

There are two theories about Bastian's motive: first, that it lay within the relationship, a product of his fears about his own future and Kelly's fate without him. The second possibility lies in the past: that Bastian feared the exposure of a secret he could not face, a secret that lay in the tangled relationships between the former East Germany and the Federal Republic that he had served as a soldier.

This theory is fuelled by what is known of the couple's last contacts. On 30 September 1992 they returned to Bonn after an absence of nearly three weeks. Among the messages waiting for them was one from Lukas Beckmann, a senior figure in the Green Party. Beckmann wanted Bastian and Kelly to complete the documentation required to see the files that had been held on them by the East German secret police - the Stasi.

The Green Party had discovered that one of its senior politicians had been a Stasi collaborator, and was planning a seminar to explore the role of the Greens in the complex web of relations between the two Germanies. For that, they needed the documents.

On the afternoon of 1 October, Bastian returned Beckmann's call. Beckmann was too busy to talk, but he passed on a message that the paperwork was urgent. That, as far as the Bonn police know, was the last telephone call Bastian made.

After the couple's deaths, Kelly's family obtained her Stasi file. Bastian's family did not apply for his. They do not believe, according to his son, Till, that the file contains anything untoward, and they have little interest in seeing it. But in the aftermath of German unification, the opening and (sometimes selective) use of Stasi files has destroyed many reputations. And in the months that followed his death, Gert Bastian's reputation was to come under hostile scrutiny.

The 115 miles of documentation that remain of the Stasi archive are housed in a former secret police building in what used to be East Berlin. Prodigious though the volume of material is, it is incomplete: some files were shredded by the Stasi themselves towards the end of the regime; some were destroyed when the headquarters was stormed in November 1989 and, more controversially, external espionage files were allegedly destroyed during the interregnum between the fall of the Wall and the 1990 elections that led to unification. There are rumours that copies exist, which the German government refuses to confirm or deny. And there is another source of information on the Stasi's pre-unification activities: its own former officers. It was from one of them that the first shots at Bastian's reputation came.

Gunther Bhnsack is a large, jovial character who now describes himself as a journalist. In his previous career he spent 26 years in the Active Measures Department of East German intelligence. Active Measures was the Stasi's psychological warfare department, whose task it was to influence the political development of the enemy, principally West Germany. 'Psychological warfare,' said Bhnsack, 'covers a multitude of activities - subversion, putting pressure on people to influence them, discrediting people, spreading rumours, truths and half-truths.'

It was in the early Eighties that Bastian strayed into Bhnsack's sights. It was a time of protest in West Germany against Nato's 1979 decision to modernise its nuclear arsenal in Europe, the so-called 'double-track' decision. The planned deployment generated a huge wave of protest and the Peace Movement was gaining in strength and influence across the country.

For the East Germans the Peace Movement was both an opportunity and a risk. The opportunity was that the citizens of West Germany could make the Warsaw Pact's case against Nato deployment; the risk was that the movement might also criticise the Warsaw Pact. In 1981, the Stasi outlined the policy its operatives should pursue: officers were instructed to give material and political support to the movement but not to attempt to lead it directly. The Stasi believed it had enough pro-East German organisations within the Peace Movement to keep the Warsaw Pact's objectives to the fore.

By this time, Bastian had left the German army and joined the Peace Movement, inspired by his fear that the deployment of a new generation of missiles in Germany made nuclear war more likely. The usefulness of such figures as Bastian to the East German case was pointed out in the Stasi instructions: 'Members of the Bundeswehr (the West German armed forces) who question the sense of the planned armament measures are to be won over.'

That was Bhnsack's department. In the early Eighties, a group of former Nato generals created an organisation called Generals for Peace which campaigned for nuclear disarmament. Because its members spoke with the authority of their previous careers, it was a formidable propaganda force. But according to Bhnsack, Generals for Peace was conceived, organised and financed by the Stasi.

'The Russians told us,' he said, 'that we should find people in the West who would implement our ideas and tactical demands and would spread them in the West. The most obvious thing to do was to look around for soldiers who could speak competently about peace and war and, since soldiers can only talk after they have retired, we had the idea of looking for people who were retired and who wanted to be politically active. That's why we looked closely at Nato and that's when we came across General Bastian in Germany and the others.'

The Stasi located 10 generals and decided to publish a book to which they would all be invited to contribute. 'Out of this rather loose gathering,' said Bhnsack, 'grew a real movement. People telephoned each other, organised debates, talked to each other. This created a real power that was in line with Moscow's ideas and we always controlled this through our intelligence services in Moscow and East Berlin.'

The operation was so sophisticated that Bhnsack said he was still not sure how much the generals appreciated that their organisation was financed from behind the Iron Curtain. 'There was a whole range of expenses which were paid jointly by Moscow and the GDR. I believe that some of the generals asked about the origin of the funds, and they asked very loudly at times, but one Dutch general replied that wherever it came from it served a good purpose.'

After German unification, the West German authorities began to investigate past Stasi activities to determine who should be prosecuted for treason. Bhnsack was interviewed about Generals for Peace.

'They thought initially that Bastian was part of a Communist spy ring,' said Bhnsack, 'and that the generals' movement was controlled by the intelligence service.' Had evidence been found to support that view, Bastian would have faced treason charges. But Bhnsack believes the generals were manipulated by the movement and not an intelligence plot.

The state prosecutor's office denies that it investigated Bastian for treason, though Bhnsack insists that it did, and the director of the authority that holds the Stasi archives confirms that he received a request for information on Bastian. Bastian was never questioned, and it is likely that his dubious role in the early Eighties might have passed unnoticed - had he not murdered Kelly and then committed suicide. But his mysterious death raised questions that have not been answered.

It seems unlikely that Bastian was a spy for the East. But did he, at the end of his life, realise the extent to which he had played the 'useful idiot'? And did he prefer to die rather than face that revelation?

(Photographs omitted)