When three of former East Germany's best-known dissidents died within a few months of each other, of similar rare forms of leukaemia, suspicions were aroused among their friends that this was more than just a coincidence.

The BBC's Terry Stiastny: Many prisoners were tortured by the Stasi in underground interrogation cells

The writer Juergen Fuchs was convinced before his death this month that he had been deliberately exposed to high levels of radiation by the East German secret police, the Stasi, which could have caused his terminal cancer.

Now, the Berlin prosecutors' office is investigating his death, but they are still missing vital evidence as to whether his claims were true.

Memorial to the tortured

Many of those dissidents were held for interrogation, and later served some of their sentences, in H�hensch�nhausen prison in the north-east suburbs of Berlin.

Today, the prison has been kept as a memorial to those who were detained, and sometimes tortured there.

Although it is no longer a prison, it has lost nothing of its power to scare the visitor arriving at its steel gates.

Former inmates guide visitors around the underground cells, and describe how they were kept, isolated and disoriented, knowing nothing of what was going on in the outside world.

Many of those former dissidents, who had been imprisoned for such "crimes" as trying to leave the country, or telling political jokes, believe it would have been entirely possible for the Stasi to have used radiation as a cruel and invisible means of punishment.

Radiologists are sceptical of the claims

In 1989, when the Berlin Wall came down, members of East German citizens' committees occupied prisons across the country.

Some of them were surprised to find powerful X-ray equipment there - not, as you might expect, in the medical centres, but in the rooms where prisoners had their photographs taken.

Now, the fear is that those "photographs" were in fact a way of exposing the prisoners to levels of radiation that were high enough to cause cancer.

The German authorities are now looking into the allegations that this was a deliberate policy by the Stasi.

Poison and sabotage

Thomas Auerbach works as a scientific researcher for the main authority investigating the Stasi. He saw X-ray equipment himself when he was taking part in a sit-in in an East German prison.

He has seen the documents which show that a variety of experiments were being carried out involving the potential uses of radiation as a means of poison and sabotage.

He believes that instead of X-rays, it was highly likely that radioactive isotopes were used to try to induce cancer in prisoners.

But so far, his searches through the archives have not brought conclusive proof.

Medical opinion is more sceptical.

Radiologists say that high doses of radiation, whether from X-rays or from more concentrated sources, have been shown to increase the risk of cancers like leukaemia.

But, on the other hand, they argue that it would be hard to administer such doses without the unwilling "patient" being aware of them or feeling side-effects.

But perhaps the hardest thing to explain is why anyone would have wanted to inflict such suffering, even on people they saw as enemies.

Ten years after the collapse of communism, the enmity that the secret police might have felt towards those who defied them is hard to imagine.