Physicians for Human Rights (PHR) issued its six-page white paper after shocking details about the range of techniques used by interrogators, including the simulated drowning known as waterboarding, came to light one week ago with release of a 2004 CIA inspector general's report.
The revelations have fueled the raging debate over interrogations during the "war on terror" of the George W. Bush administration.
"Health professionals played a central role in developing, implementing and providing justification for torture," PHR said in its report.
"Health professionals in the (federal government) and psychologist contractors engaged in designing and monitoring harmful interrogation techniques. Such medical participation in torture is a clear violation of medical ethics."
The not-for-profit group also said the medical experts "were complicit in selecting and then rationalizing these abusive methods, whose safety and efficacy in eliciting accurate information have no valid basis in science."
Interrogators are alleged to have used a range of techniques on suspected Al Qaeda members, including physical threats, mock executions, choking to the point where detainees lost consciousness and even using a stiff brush to scrub a detainee's skin raw.
PHR warned that such spy agency techniques - and monitoring by doctors to gauge their effectiveness -- "approaches unlawful experimentation" on human subjects.
The report's lead author, PHR medical advisor Scott Allen, pulled no punches in a statement on the organization's website, saying "medical doctors and psychologists colluded with the CIA to keep observational records about waterboarding, which approaches unethical and unlawful human experimentation."
By standing by to monitor the effects of acts such as waterboarding on detainees, the health professionals were "laying a foundation for US government lawyers to rationalize the CIA's illegal torture program."
PHR urged a separate investigation of the medical experts involved to determine whether "criminal and unprofessional conduct" took place, and said those who violated medical ethics rules should lose their license.