The BBC's David Eades examines the evidence

Thousands of Muslim men and boys were massacred as they fled from the United Nations safe area of Srebrenica in 1995.

The former Yugoslav army's military handbook explains how best to use BZ

It is alleged that a gas called BZ was used by the former Yugoslav army to incapacitate the Bosnian Muslims before they were killed.

The gas is a non-lethal chemical agent causing a range of physical and mental symptoms in its victims, including hallucination and disorientation as well as stupor and coma.

With mass graves still being uncovered in the region, the human rights organisation, Human Rights Watch, has gathered evidence from the few survivors.

Their accounts of what happened to them as they escaped through the countryside point to the use of BZ.

BZ symptoms include hallucination and disorientation

Several eyewitnesses said they saw clouds of yellowish smoke hanging in the air.

One man told how his brother began tearing at his hair and clothing begging for water. He died hours later.

Others told of friends and colleagues acting wildly out of character and hallucinating.

Another recounted how one group stood together and embraced before exploding a grenade that killed them all.

'Could be stress'

So far, the evidence gathered is circumstantial and some forensic experts put the behaviour down to the enormous levels of fear and stress the men were subjected to.

But chemical weapons specialists say if they could examine exploded shells from the region they could prove whether or not BZ was used.

Dr Jan Medema said: "If you went down there and had a group of chemical experts taking samples of the remains of the shells that were fired then you could determine from the remains whether there were any chemicals used in those weapons."

Human Rights Watch says it has gathered several pieces of clear evidence to show that the former Yugoslav army used BZ.

One such item is the former Yugoslav army's own military handbook explaining how best to use BZ.

The evidence is now with prosecutors of the war crimes tribunal, who will decide whether to continue the investigation.