US government officials may have conspired to conceal evidence that three Guantanamo Bay inmates could have been murdered during interrogations, according to a six-month investigation by American journalists.

All three may have been suffocated during questioning on the same evening and their deaths passed off as suicides by hanging, the joint investigation for Harper's magazine and NBC News has concluded.

The magazine also suggests the cover-up may explain why the US government is reluctant to allow the release of Shaker Aamer, the last former British resident held at Guantanamo, as he is said to have alleged that he was part-suffocated while being tortured on the same evening.

“The cover-up is amazing in its audacity, and it is continuing into the Obama administration,” said Scott Horton, the contributing editor for Harper's who conducted the investigation. When the three men — Salah Ahmed al-Salami, 37, a Yemeni, and two Saudis, Talal al-Zahrani, 22, and Mani Shaman al-Utaybi, 30 — died in June 2006, the camp's commander declared that they had committed suicide and that this had been “an act of asymmetrical warfare”, rather than one of desperation.

According to an official inquiry by the US navy, whose report was heavily censored before release, each man was found in his cell, hanging from bedsheets, with their hands bound and rags stuffed down their throats.

However, Horton spoke to four camp guards who alleged that when the bodies were taken to the camp's medical clinic they had definitely not come from their cell block, which they were guarding, and appeared to have been transferred from a 'black site', known as Camp No, within Guantanamo, operated by either the CIA or a Pentagon intelligence agency.

The men said that the following day, a senior officer assembled the guards and told them that the three men had committed suicide by stuffing rags down their throats, that the media would report that they had hanged themselves, and ordered that they must not seek to contradict those reports.

Harper's says that when the bodies of the three men were repatriated, pathologists who conducted post-mortem examinations found that each man's larynx, hyoid bone and thyroid cartilage — which could have helped determine cause of death — had been removed and retained by US authorities.

— The Guardian, London