US government researchers must have known those were unethical studies, while they deliberately infected Guatemalan prison inmates and mental patients with syphilis for an experiment in the 1940s, according to a US presidential commission.

The Presidential Commission for the Study of Bioethical Issues is holding public hearings on Monday and Tuesday to unveil the full details of their investigation into the controversial program, which was intended to evaluate the effectiveness of penicillin in treating sexually transmitted diseases.

In those studies, researchers backed by the US Public Health Service used infected prostitutes and direct injections to transmit the disease to nearly 700 Guatemalans from 1946 to 1948.

The commission concluded that the scientists used no informed consent procedures and the researchers failed to act in accordance with "minimal respect of human rights" and that the study was sloppily done and ethically objectionable.

Besides, the commission said the investigators deliberately kept their actions secret from people in the trial as well as the US and Guatemalan scientific communities.

"These researchers knew these were unethical experiments, and they conducted them anyway," said Raju Kucherlapati of Harvard Medical School, a commission member. "That is what is reprehensible."

The commission found that at least 5,500 prisoners, mental patients, soldiers and children were drafted into the experiments, including at least 1,300 who were exposed to the sexually transmitted diseases syphilis, gonorrhea and chancroid.

"This is a dark chapter in our history. It is important to shine the light of day on it. We owe it to the people of Guatemala who were experimented on, and we owe it to ourselves to recognize what a dark chapter it was," said Amy Gutmann of the University of Pennsylvania, the commission's chairwoman.

For decades, the experiments remained secret until they were uncovered last October by Wellesley College professor Susan Reverby who was conducting research on the Tuskegee syphilis study, a similarly controversial experiment carried out from 1932 to 1972 in which scientists infected poor black farmers in Alabama with the disease.

Last year, Guatemalan President Alvaro Colom described the 1940's study as a crime against humanity. US President Barack Obama offered his apologies for the study. US Secretary of State Hillary Clinton and Secretary of Health and Human Services Kathleen Sebelius also issued a joint statement condemning it.

"Although these events occurred more than 64 years ago, we are outraged that such reprehensible research could have occurred under the guise of public health," Clinton and Sebelius wrote. "We deeply regret that it happened, and we apologize to all the individuals who were affected by such abhorrent research practices."