By Mark Dummett
BBC News, Dhaka
Guerrillas attack Pakistani militiamen in newly-independent Bangladesh
As Bangladesh's bloody war of independence from Pakistan came to its end, Dr MA Hassan went in search of his brother.
He was afraid that Selim, who like him was an officer in the pro-liberation forces, had been killed in one of the last battles of the conflict, and he wanted to recover his corpse.
He didn't find it, but as he stumbled through a marsh at the northern edge of Dhaka, he came across a horrific scene.
"That day, 31 January 1972, I saw a few hundred bodies, mutilated dead bodies, littered all around that place," he recalled. "There were marks of torture on every body; nails turned out, eyes gouged out, hearts taken out."
He added: "Some were female, their breasts were amputated, private parts mutilated. I had to push the bodies one by one to make my way. Mostly they were the innocent public."
At that time, hundreds of other mass graves were also being discovered across the newly independent country. This followed a nine-month war when the Pakistani army tried to bludgeon the citizens of its eastern province into renouncing their dreams of self-rule.
The crisis was precipitated when East Pakistanis (who later became Bangladeshis) voted overwhelmingly in favour of autonomy and West Pakistan responded by sending in its army.
Hundreds of thousands of people were killed, including Hindus, political activists, intellectuals and students. The Pakistani army carried out "collective punishment" where they suspected villagers of helping the freedom fighters.
Thousands of women were raped, millions fled into India. Bangladeshis say the killings amounted to a genocide and that three million people died.
Thirty-six years later, Dr Hassan took me back to the place where he had come across the corpses, an area called "Black Water". It is one of the wet wastelands that ring the Bangladeshi capital and life there is now perfectly normal, if bleak. When we visited, men were smashing bricks into chips to help build a new road, and women and children were washing in a pond.
Dr Hassan revisits the scene of the killings
There is no memorial to the hundreds of people killed there and none of the killers has ever been brought to justice. But what he witnessed has inspired Dr Hassan to do something about that.
He is a leading member of the War Crimes Fact Finding Committee which is dedicated to investigating the massacres and putting pressure on the government to hold war crime trials.
Although most killings were carried out by the Pakistan army, many locals helped them.
These collaborators became members of so-called peace committees, or armed militia of razakars (volunteers).
In one of the most notorious incidents of the war, more than 150 academics and journalists (including BBC reporter Nizamuddin Ahmed) were rounded up in Dhaka on the eve of Pakistan's defeat and killed by members of a group call Al-Badr, which was allegedly made up of members of the religious party Jamaat-e-Islami.
At the end of the war hundreds of alleged collaborators were arrested, and many were executed by pro-liberation forces.
But Bangladesh's leader, Sheikh Mujibur Rahman, then granted a general amnesty and subsequent governments shied away from confronting such a controversial issue.
The War Crimes Fact Finding Committee is now at the forefront of a campaign for justice, which has gathered momentum in Bangladesh since a military-backed interim government took over in January 2007. The campaigners have been encouraged by the government's promise of political reforms.
That is because this is now a deeply political issue. Many of the people accused of committing war crimes have gone on to become influential public figures. Jamaat-e-Islami has gone from being a fringe party in 1971, to a junior coalition partner in the last elected government.
The campaigners are demanding that the authorities block Jamaat from standing in the next elections to be held in December.
Matiur Rahman Nizami, leader of Jamaat-e-Islami
None of the accusations against them are new. Reporters covering the war for newspapers such as The Times of London, and the New York Times, wrote at that time that Al-Badr comprised Jamaat members.
The War Crimes Fact Finding Committee has spent the last 19 years gathering reams of documents and eyewitness accounts to back up their claims, and has handed them over to the government, along with the names of 1,150 alleged war criminals.
But Jamaat-e-Islami, which describes itself as a "moderate Islamic political party that believes in democracy and human rights" says it is the victim of a political vendetta. None of its leaders has ever been prosecuted for their alleged activities during the war and its lawyer Abdur Razzak says the accusations are baseless.
"In this country the law of defamation has become totally ineffective," he said. "If I say you are a war criminal there is nothing you can do about it. This is being used against Jamaat-e-Islami for a political purpose.
"We did not take part in any of the crimes that has been alleged against us.
"Had there been any specific allegations, there would have been prosecutions in the last 36 years."
But Dr Hassan, who has received death threats since publishing the list of alleged war criminals, denies he has a political agenda. He says he doesn't want to "take revenge, but to break the silence of impunity".
Some of the campaigners worry that that silence will never be broken, and that unless war crime trials establish the truth soon, then there is a danger that the history of Bangladesh's cruel birth will be rewritten.
In response, a group of bloggers has now started posting archives on the web so that anyone with an internet connection can discover for themselves what happened.