America’s professional association of psychologists has quietly declined to rebuke one of its members, a retired US army reserve officer, for his role in one of the most brutal interrogations known to have to taken place at Guantánamo Bay, the Guardian has learned.

The decision not to pursue any disciplinary measure against John Leso, a former army reserve major, is the latest case in which someone involved in the post-9/11 torture of detainees has faced no legal or even professional consequences.

In a 31 December letter obtained by the Guardian, the American Psychological Association said it had “determined that we cannot proceed with formal charges in this matter. Consequently the complaint against Dr Leso has been closed.”

But the APA did not deny Leso took part in the brutal interrogation of the suspected 20th 9/11 hijacker, Mohammed al-Qahtani, whose treatment the Pentagon official overseeing his military commission ultimately called “torture”.

Leso was identified as “MAJ L” in a leaked log, published by Time magazine in 2005, of Qahtani’s marathon interrogation in November 2002. With Leso recorded as present for at least some of the session, Qahtani was forcibly hydrated through intravenous drips and prevented from using the bathroom until he urinated on himself, subjected to loud music, and repeatedly kept awake while being “told he can go to sleep when he tells the truth”.

At one point, Qahtani was instructed to bark like a dog.

“Dog tricks continued and detainee stated he should be treated like a man,” the log records. “Detainee was told he would have to learn who to defend and who to attack.”

During an interrogation on 27 November 2002, the log records a direct intervention by Leso: “Control puts detainee in swivel chair at MAJ L’s suggestion to keep him awake and stop him from fixing his eyes on one spot in booth.”

The APA’s move concludes a years-long effort within the organization to get the association to condemn members who took part in torture. Those who argued for censuring Leso said that the organization has opened the door to future wartime violations of its central do-no-harm ethos.

“With Leso, the evidence of his participation is so explicit and so incontrovertible, the APA had to go to great lengths to dismiss it,” said Steven Reisner, a New York clinical psychologist who unsuccessfully ran for the APA presidency last year. “The precedent is that APA is not going to hold any psychologist accountable in any circumstance.”

Trudy Bond, an Ohio psychologist who filed the complaint against Leso, cited APA’s policy on interrogations and torture as she said the organization had sent the message that “psychologists are free to violate our ethical code, perhaps, in certain situations”.

The APA’s communications chief, Rhea Farberman, told the Guardian that a seven-year ethics investigation could not meet the burden of finding “direct unethical conduct” by Leso, and said it was “utterly unfounded” to fear the organization has condoned professional impunity.

“A thorough review of these public materials and our standing policies will clearly demonstrate that APA will not tolerate psychologist participation in torture,” Farberman said.

Documents that emerged from a Senate armed services committee torture inquiry detailed Leso’s involvement in an early “Behavioral Science Consultation Team” at Guantánamo, which was instrumental in crafting torture techniques out of measures taught to US troops to withstand brutal treatment.

Leso, whose name is redacted in a lengthy report produced by the committee in 2008, helped write a memorandum in October 2002, “Counter-Resistance Strategies”, for Guantánamo staff who were under pressure from the chain of command to produce intelligence from the detainee population.

The memorandum detailed the use of abusive conditions and techniques on the detainees, including isolation, “stress positions”, sensory and sleep deprivation, dietary manipulation and exposure to extreme cold. Those techniques migrated through the Pentagon bureaucracy and were ultimately used at Abu Ghraib prison in 2003.

“Counter-Resistance Strategies” also recommended manipulating the living conditions of detainees outside the interrogation chambers, such as limiting “resistant” detainees to four hours of sleep daily, depriving them of “comfort items” like sheets and mattresses and controlling access to their Qur’ans.

“All aspects of the [detention] environment should enhance capture shock, dislocate expectations, and support exploitation to the fullest extent possible,” according to a section of the memorandum published by the Senate committee.

The minutes of an October 2002 Guantánamo meeting published by the committee identified Leso by name, rank and membership in the Behavioral Science Consultation Team.

But those minutes and other records published by the Senate also portray Leso as being at least ambivalent about detainee abuse. “Force is risky, and may be ineffective due to the detainees’ frame of reference. They are used to seeing much more barbaric treatment,” the minutes record him saying.

Leso and a psychiatrist colleague on the team, Major Paul Burney, conceded in Counter-Resistance Strategies that torture was a poor method of extracting accurate information.

“Experts in the field of interrogation indicate that the most effective interrogation strategy is a rapport-building approach. Interrogation techniques that rely on physical or adverse consequences are likely to garner inaccurate information and create an increased level of resistance … The interrogation tools outlined could affect the short term and/or long term physical and/or mental health of the detainee,” they wrote.

Leso and Burney told the Senate panel in 2007 interviews that they were “not comfortable with the memo they were asked to produce”, according to the Senate report.

That stated discomfort appears to have influenced the APA in exonerating Leso.

“Available evidence in the public domain also includes that, in the face of pressure from the highest levels of the Bush administration which strongly supported ‘enhanced’ interrogation tactics, the respondent sought consultation and argued against such approaches and in favor of rapport-building approach,” wrote Lindsay Childress-Beatty, the deputy director of the APA’s ethics office, on 31 December.

The APA also found it exculpatory that Leso “did not request to become involved with detainee interrogations but was rather informed that he would be in the role of behavioral science consultant (“BSC”) only after he arrived in Guantánamo Bay in the summer of 2002”.

Attempts at contacting Leso, who retired from the military in 2004, were unsuccessful. A woman who answered the phone at a residence listed under his name said Leso no longer resided there. Another woman who identified herself as a relative of Leso’s at a different phone number said she did not have a way of contacting him. The APA said it had no contact details it could share with the Guardian.

Bond and other psychologists said they considered the APA’s response to be a whitewash of Leso’s record, focusing on his subsequently vocalized doubts, rather than his participation in the interrogation.

“He should have refused,” Bond said. “If the psychologists had done this back in 2002, things might have been different.”

Stephen Soldz, a Boston-based psychologist, called Leso’s involvement in torture “probably the clearest, most documented case [of psychologist] participation in abuse that we’re going to have.”

“This is the most there’s ever going to be, and for many years APA has been shouting from the top of its lungs, ‘Give us names and we’ll bring people up on ethics charges,’ and yet they say the evidence is insufficient,” Soldz said.

Qahtani’s lawyer, Shayana Kadidal of the Center for Constitutional Rights, said it was “astonishing” that the APA concluded Leso pushed back against torture.

“The same memo warned that the practices Leso came up with could produce 'irreversible' psychological harm, yet Leso went ahead and took part in interrogation sessions that applied those same techniques. He was in the room, watching as our client collapsed psychologically from being degraded, physically abused, deprived of sleep and human contact. And we’re supposed to believe some throwaway line in a memo written a few weeks earlier excuses all of that and makes Leso fit to serve as a psychologist treating vulnerable patients?” Kadidal said.

The APA’s official position is an “unequivocal condemnation” on “all techniques considered torture or cruel, inhuman or degrading treatment or punishment” under the relevant international treaties. Banned techniques include some listed in Leso’s 2002 memo, including isolation, hooding, dietary manipulation, sleep deprivation, and stress positions.

But in its 31 December letter dropping Leso’s case, the APA said that Leso’s involvement in US interrogations predated its opposition to such practices. “APA did not issue its first policy on interrogations until three years later, in 2005,” its deputy ethics director, Childress-Beatty, wrote.

Qahtani, who claimed that he lied to his interrogators to end the abuse, remains at Guantánamo Bay, caught in a legal limbo caused in part by his torture.

The Pentagon twice charged Qahtani with war crimes related to 9/11 in 2008. But the Defense Department official overseeing his military tribunal, retired federal judge Susan Crawford, ruled she could not proceed with the case, since the government “tortured Qahtani”.

“The techniques they used were all authorized, but the manner in which they applied them was overly aggressive and too persistent,” Crawford told the Washington Post in early 2009.

Leso is the latest case in which US officials involved with torture have escaped legal or professional consequences. A justice department investigation into CIA torture resulted in no indictments, and it never considered examining the architects of torture policy. Nor has torture caused its architects to suffer professionally: some have returned to tenured academic positions, awarded federal judgeships and sit on the boards of major corporations.

Some experts are concerned that the lack of legal or disciplinary consequences for those involved in torture will encourage a future president to lift President Barack Obama’s executive order banning torture after another terrorist attack. Bond and other psychologists worry their colleagues will aid brutal interrogations.

The APA’s Farberman said that was a baseless fear. “The concern that APA’s decision to close the matter against Dr John Leso will set a precedent against disciplining members who participate in abusive interrogations is utterly unfounded. Each ethics complaint is carefully reviewed on its own merits,” she said.