For more than an hour, the bipartisan group, which included future-House Speaker Nancy Pelosi, was given a virtual tour of the CIA’s detention sites and the harsh techniques interrogators had devised to make their prisoners talk.

Among the techniques described, said two officials present, was waterboarding, a practice that years later would be condemned as torture by Democrats and some Republicans on Capitol Hill. But on that day, no objections were raised. “The briefer was specifically asked if the methods were tough enough,” said a US official, who witnessed the exchange.

Waterboarding as an interrogation technique has its roots in some of history’s worst totalitarian nations, from Nazi Germany and the Spanish Inquisition to North Korea and Iraq. In the US, the technique was first used five decades ago as a training tool to give US troops a realistic sense of what they could expect if captured.

Long before “waterboarding” entered the public discourse, the CIA gave key legislative overseers about 30 private briefings, some of which included descriptions of that technique and other harsh interrogation methods.

No formal objections were raised by the lawmakers briefed about the harsh methods during the two years in which waterboarding was employed, from 2002 to 2003, said Democrats and Republicans.

Congressional officials say the groups’ ability to challenge the practices was hampered by strict rules of secrecy that prohibited them from being able to take notes or consult legal experts or members of their own staffs.

“In fairness, the environment was different then because we were closer to September 11,” said a US official present during the briefings. “But there was no objecting, no hand-wringing. The attitude was, ‘We don’t care what you do to those guys as long as you get the information you need to protect Americans’.”

GOP lawmakers and Bush administration officials previously said members of Congress were well-informed and supportive of the CIA’s use of harsh interrogation techniques. But the details of who in Congress knew what, and when about waterboarding have not previously been disclosed.

In September 2006, the CIA for the first time briefed all members of the House and Senate intelligence committees, producing some heated exchanges with CIA officials, including Director Michael Hayden. The CIA director said during a TV interview two months ago that he had informed congressional overseers of “all aspects of the interrogation programme”.

The US military has officially regarded the tactic as torture since the Spanish-American War. In general, the technique involves strapping a prisoner to a board or other flat surface, and then raising his feet above the level of his head. A cloth is then placed over the subject’s mouth and nose, and water is poured over his face to make the prisoner believe he is drowning.

During these sessions, the agency provided information about the techniques it was using as well as the information it collected.

In May 2007, four months after Democrats regained control of Congress, four senators submitted written objections to the CIA’s use of that tactic and other, still unspecified “enhanced” techniques in two classified letters to Hayden last spring.