The US research project in which government researchers deliberately infected Guatemalan prison inmates and mental patients with syphilis in the 1940s has been described as an "institutional failure" by a US presidential commission.
Nearly 5,500 people were subjected to diagnostic testing and more than 1,300 were exposed to venereal diseases by human contact or inoculations in research meant to test the drug penicillin, the commission said on Monday.
Within that group, "we believe that there were 83 deaths," said Stephen Hauser, a member of the commission which has poured over 125,000 documents linked to the episode since being set up by President Barack Obama last November.
However, Hauser said not enough evidence existed to confirm that the procedures the people endured during the study was what had killed them.
Among the 1,300 people exposed to STDs during research between 1946 and 1948, "under 700 received some form of treatment as best as could be documented," he said.
Guatemala's vice-president Rafael Espada told Al Jazeera that Guatemalan doctors were also present during the tests.
Obama personally apologised to Guatemalan President Alvaro Colom in October before ordering a thorough review of what happened.
Secretary of State Hillary Clinton described the experiments as "clearly unethical".
The experiments are already considered one of the darkest episodes of medical research in US history, but panel members say the new information indicates that the researchers were unusually unethical, even when placed into the historical context of a different era.
Commission president Amy Gutmann called it an "historic injustice," and said the inquiry aimed to "honour the victims and make sure it never happens again".
"It was not an accident that this happened in Guatemala," Gutmann said. "Some of the people involved said we could not do this in our own country."
The US researchers "systematically failed to act in accordance with minimal respect for human rights and morality in the conduct of research," Gutmann said, citing "substantial evidence" of an attempted cover-up.
A Guatemalan study, which was never published, came to light in 2010 after Wellesley College professor Susan Reverby stumbled upon archived documents outlining the experiment led by controversial US doctor John Cutler.
They revealed that some of the experiments were more shocking than was previously known.
For example, seven women with epilepsy, who were housed at Guatemala's Asilo de Alienados (Home for the Insane), were injected with syphilis below the back of the skull, a risky procedure.
The researchers thought the new infection might somehow help cure epilepsy. The women each got bacterial meningitis, probably as a result of the unsterile injections, but were treated.
'Crimes against humanity'
Perhaps the most disturbing details involved a female syphilis patient with an undisclosed terminal illness. The researchers, curious to see the impact of an additional infection, infected her with gonorrhoea in her eyes and elsewhere. Six months later she died.
Cutler and his fellow researchers enrolled 1,500 people in Guatemala, including mental patients, for the study, which aimed to find out if penicillin could be used to prevent sexually transmitted diseases.
Initially, the researchers infected female Guatemalan commercial sex workers with gonorrhoea or syphilis, and then encouraged them to have unprotected sex with soldiers or prison inmates.
Neither were the subjects told what the purpose of the research was nor were they warned of its potentially fatal consequences.
Cutler, who died in 2003, was also involved in a highly controversial study known as the Tuskegee Experiment in which hundreds of African-American men with late-stage syphilis were observed but given no treatment between 1932 and 1972.
The Guatemalan president has called the 1946-1948 experiments conducted by the US National Institutes of Health "crimes against humanity" and ordered his own investigation.