The investigation by the Guardian and the BBC into direct Pentagon involvement in the systematic torture of Sunni insurgents in Iraq is a bloody reminder of the catastrophe that the 2003 invasion wreaked on the people of Iraq. It also a key reason behind the decade of sectarian violence the war has left in its wake.

After a decade of the most extreme bloodshed on both sides, the Sunni minority is now asserting its collective muscle in an organised fashion under the leadership of figures such as the Sunni scholar Abdul-Malik al-Saadi. The immediate reason for this upsurge in confidence among the Sunnis of Iraq is not hard to find.

The rebellion in neighbouring Syria, which began as essentially secular resistance movement, has attracted Sunni extremist groups from across the globe in support of the effort to bring down President Assad. Armed by regional troika of Turkey, Saudi Arabia and Qatar, they are now about to be provided with military support by the west, including Britain, in an echo of the strategy under which western countries provided firepower to support the Islamist rebel forces in Libya.

This, in turn, has emboldened the Sunni minority, comprising a fifth of Iraq's population, which has been holding large-scale public demonstrations. Their attempt to mount a cross-sectarian challenge to the government in Baghdad has also attracted the support of Shia cleric Muqtada al-Sadr. Meanwhile, the remnant of al-Qaida in Iraq has been attempting to use the protests as cover for a highly incendiary campaign inciting Sunnis to take up arms against the regime.

The essentially pluralist administration in Baghdad is committed to the re-intergration of the Sunni minorities into positions of responsibility in Iraq. They have set up a dedicated ministry of reconciliation, and the minister for human rights is actively pursuing an agenda for positive change. My UN mandate is to work closely with the government and civil society in Iraq, aiming to delivering cross-sectarian initiatives that will stem the flow of violence.

But who is really to blame? The government is in no doubt that the causes of the deep-seated sectarian violence in the country lie in the excessive and extreme policy of de-Ba'athification pursued by the US administration under the now discredited Paul Bremmer.

During the Saddam era, membership of the Ba'ath party was effectively a prerequisite for public employment in positions of any responsibility. Expelling all Sunni members of the Ba'ath party from the administration was as ill-judged a policy as excluding members of the Communist party from public office in the former Soviet Union following the collapse of the Berlin wall. Individuals who had no connection whatsoever with the crimes of the former regime were ignominiously put out of their jobs and often their homes.

Almost overnight, a privileged Sunni ruling class was turned into a marginalised and unemployed minority, with a deep sense of grievance against the US-backed Shia authorities. Many were men with military or paramilitary training, access to weapons, and nothing to to do except hate.

Into this political tinderbox, the Bush-era Pentagon, the CIA, and their proxies among the brutal Shia militias, threw the lighted match of systematic torture. Suspected Sunni insurgents were rounded up and subjected the most brutal forms of torture under the eyes of American agents. The Guardian/BBC investigation advances our knowledge of this criminal conspiracy, taking it right to the heart of the Bush administration. That set off a chain reaction that is still reverberating in Iraq.

On Tuesday this week, I presented a report to the UN Human Rights Council in Geneva calling on the US and other states, including the UK, to secure accountability for the crimes committed by the Bush-era CIA and its allies in pursuit of the counterproductive campaign of rendition, secret detention and torture. To the list of international crimes committed by that administration must now be added the evidence uncovered by the Guardian and the BBC.

As long ago as 2006, during its last periodic review by the UN Human Rights Committee, the US was heavily criticised for adopting a policy of impunity towards the officials who committed these grave and systematic crimes. It is due for its next periodic review in the autumn of this year, and I have every confidence that the committee will expect to see the results of a full investigation into these new allegations. This would have the objective of bringing those responsible, including the politicians who authorised this conduct, to justice.

Taken with the compelling evidence that is now available concerning the crimes of torture and rendition that were committed internationally, this latest investigations presents an image of lawlessness and hypocrisy that is antithetical to building international co-operation with the Islamic peoples of the Middle East and North Africa. The urgent and imperative need to develop an international consensus in favour of ethical counter-terrorism was underlined by William Hague in a recent speech on ethical to the Royal United Services Institute in London, in which he said that where allegations of this kind are made, they must be fully investigated and the perpetrators brought to justice. One can only hope that he will be impressing upon the US Department of Justice the need for an investigation into the allegations against David Petraeus and others.

Failure to address the past inevitably generates the misperception that the perpetrators remain as beneficiaries of official toleration or collusion. However inaccurate some of these perceptions may be, they will endure until decisive action is taken. Holding those responsible to account is now the only way of genuinely drawing a line under the past.

Iraq is in desperate need of reconciliation initiatives. There may well be a case for an effective truth and reconciliation commission. But before reconciliation, there must be reckoning with the past.

Justice for the perpetrators of these crimes is an essential prerequisite to peace and stability in the region.