As the US presidential election reaches a climax against the background of the financial crisis, another silent, dark, time bomb of an issue hangs over the two candidates: torture. For now, there seems to be a shared desire not to delve too deeply into the circumstances in which the Bush administration allowed the US military and the CIA to embrace abusive techniques of interrogation - including waterboarding, in the case of the CIA - which violate the Geneva conventions and the 1984 UN torture convention.

The torture issue's cancerous consequences go deep, and will cause headaches for the next president. New evidence has emerged in Congressional inquiries that throw more light on the extent to which early knowledge and approval of the abuse went to the highest levels. What does a country do when compelling evidence shows its leaders have authorised international crimes?

For three years I have followed a trail which leads unambiguously to the conclusion that the real bad eggs were not Lyndie England or others on the ground in Abu Ghraib, but the most senior officials in the White House, the Pentagon and the department of justice. Over recent months, Congress has been looking into the role of senior officials involved in the development of interrogation rules. These have attracted relatively scant attention; little by little, however, senators and congressmen have uncovered the outlines of a potentially far-reaching criminal conspiracy.

The first hearings were convened before the judiciary committee of the House of Representatives, at the instance of its chairman, Congressman John Conyers, apparently off the back of my book Torture Team. Parallel hearings have been held before the Senate armed services committee.

The evidence that has emerged is potentially devastating. It confirms, for instance, that the search for new interrogation techniques for use at Guantánamo began not with the local military but in the offices of Donald Rumsfeld and his chief lawyer, Jim Haynes. It shows that when the career military expressed objections on legal grounds, Haynes intervened to stop the normal process of review. And it shows a previously unknown interplay between the department of defence and the CIA: a visit to Guantánamo in September 2002 by the administration's most senior lawyers was followed days later by a senior CIA lawyer, to brief on the new techniques. "If someone dies while aggressive techniques are being used," he explained, "the backlash of attention would be severely detrimental."

Last month the Senate armed services committee received new material from Condoleezza Rice, the first cabinet-level official to confirm high-level involvement in discussions on interrogation techniques. "I participated in a number of meetings in 2002 and 2003 ... at which issues relating to detainees in US custody, including interrogation issues, were discussed," she said. Those present at such meetings included Rumsfeld, attorney general John Ashcroft, Colin Powell, Paul Wolfowitz and CIA director George Tenet. The meetings, which concerned the CIA programme, "occurred inside the White House". Rice confirmed she was aware of the existence of, but did not read, the justice department legal advice of August 1 2002 that abandoned the international definition of torture and replaced it with a definition drawn from a US Medicare statute.

Buried away in this testimony lies the most dangerous material of all: evidence which may establish that abuses on detainees in Iraq in September 2003, in the period perhaps including the events at Abu Ghraib, were the result of decisions taken at the highest levels of the administration. The administration has long proclaimed it did not allow aggressive interrogations in Iraq, since the Geneva conventions applied. Last month we learned this was false: not everyone had protection under Geneva. If you were considered to be a terrorist, you had no protection at all. A senior US intelligence officer visited Iraq in September 2003. He witnessed abusive interrogation techniques that violated Geneva and complained. The response? He was told the techniques "were pre-approved by DoD GC or higher". DoD GC is the general counsel at the department of defence, Jim Haynes. Who could be higher? His boss: Rumsfeld.

I have testified before Congress on these issues, and have been asked if there should be criminal investigations and prosecutions. At the very least, the next US president must ensure the full facts are established. It will then be for others to decide what follows. But if the US doesn't get its own house in order and restore its reputation for the rule of law, others will surely step in.

• Philippe Sands QC is professor of law at UCL, a barrister at Matrix Chambers and author of Torture Team p.sands@ucl.ac.uk