The Tuskegee syphilis experiment of the 20th century is often cited as the most famous example of unethical medical research. Now, evidence has emerged that it overlapped with a shorter study, also sponsored by U.S. government health agencies, in which human subjects were unknowingly being harmed by participating in an experiment.

Research from Wellesley College professor Susan Reverby has uncovered evidence of an experiment in Guatemala that infected people with sexually transmitted diseases in an effort to explore treatments.

The U.S. government apologized for the research project on Friday, more than 60 years after the experiments ended. Officials said an investigation will be launched into the matter.

The Tuskegee and the Guatemala studies show what National Institutes of Health Director Francis Collins called a "a dark chapter in the history of medicine."

As unethical as the methods were, the basic research questions behind both studies were highly relevant at the time, said Peter Brown, medical anthropologist at Emory University. Research in Guatemala focused on the powers of penicillin; in Tuskegee, researchers wanted to know the natural history of syphilis....