Maryland ? A federal investigation has found that heart attack survivors enrolled in a study of a controversial alternative medicine treatment were not told enough about potential dangers from the drug being tested, including death.

The study is testing chelation — infusions of a drug that in this case has been removed from the market for safety concerns. A different type of chelation is used to treat lead poisoning.

Findings from the investigation were revealed this week by the U.S. Office for Human Research Protections in a letter to the three medical centers leading the study.

The probe found that several doctors doing the study had been accused of poor practices by state medical boards or involved in insurance fraud, and that at least three are convicted felons. While “concerning,” this doesn’t prevent them from participating in federal research, the government’s letter says.

Safety issues involving the study drug were referred to the federal Food and Drug Administration to investigate.