There is a nagging splinter in the American psyche that refuses to succumb to the abrasions of passing time.

The Vietnam war ruptured a nation's consciousness, begged enormous questions of morality and killed almost 60,000 US soldiers in a humiliating military and political defeat.

The My Lai massacre, in which a US platoon slaughtered 504 Vietnamese villagers, shamed the country. And now, even as this most notorious atrocity of the war is beginning to fade into the past, a Pulitzer Prize-winning investigation has unveiled an even more horrific killing spree in what Vietnam appropriately labels the American war.

This latest chapter of barbarity unleashed on an American public coincides with the newest bout of national introspection and divide over the current occupation of Iraq, including the embarrassment of the Abu Ghraib prison abuses.

But perhaps just as startling as the details of seven months of madness by a 45-man American reconnaissance team, is that it took 37 years to be made public. The authors have unearthed a savage saga that unravelled over 1967 of civilian killings, scalpings, bags and necklaces of human ears, bags of gold teeth, army investigations and cover-ups all the way to the White House.

In November 1965, Tiger Force platoon was founded as the 101st Airborne's version of Special Forces, a unit designed to 'out-guerilla the guerillas'. The unit was small, mobile and trained to kill; to be self-sufficient and roam the beleaguered Central Highlands in an attempt to choke the Ho Chi Minh trail.

But at some point after nearly 18 months, the motley band of soldiers began to lose sight of the conventions of war, the line between enemy soldier and civilian, and embarked on an apocalyptic trail of blood, death, mutilation and depravity. Farmers were killed in their fields, men, women and children were gunned down through the feeble walls of their huts, families seeking succour in underground tunnels had grenades lobbed on to them.

Sallah and Weis write: 'After surviving almost a year of ambushes, jungle rot, booby traps and heat exhaustion, the Tigers were past the point of no return. [They] had come to the grim conclusion that the secret to surviving was to kill all Vietnamese, no matter whose side they were on.'

Repeatedly, witnesses interviewed later claimed no weapons were found after most of the killings - even though the deaths were racked up as enemy kills after 'brief engagements'. 'There were no rules and regulations any more. Half the unit had grown long, scraggly beards ... several were openly wearing necklaces of ears ... Whenever the smell of rotting flesh was too strong, [one] would toss away his current necklace and make a new one from ears he carried in a ration bag filled with vinegar.'

But not all the unit's members could stomach the blood-letting, and disturbing reports were filed which were slowly shuffled along until they showed up in the army's Criminal Investigation Command in 1972. A 41/2-year investigation followed, involving interviews with many of the unit's members, and reports to the Nixon White House.

The investigation - which concluded that hundreds of civilians had been murdered in a seven-month orgy of madness - went well past the fall of Saigon in April 1975, but was eventually shelved by the Pentagon.

For decades the case lay buried in government archives, not even known to war historians. Then by a curious twist of fate, the documents found their way to a newspaper, and forced the American public to once again confront a dark past.

'You think back and say, 'I can't believe I did that'. But now you know what you did was wrong. The killing gets to you. You just can't escape it ... It's in the middle of the night, when the demons come, that you remember,' say two Tiger Force members.

Tiger Force is a damning indictment of how America has lost the moral high ground, and a grim reminder that dark secrets rarely remain hidden forever.