(Associated Press) -- 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.

Federal officials recommended corrective steps to researchers, but have allowed the study to go on while the probe continues -- a decision that angered critics.

It is "incredibly unethical, subjecting anyone to these risks," said Arthur Caplan, bioethics chief at the University of Pennsylvania. "This study should not be going on."

The study is sponsored by the National Heart, Lung and Blood Institute and the National Center for Complementary and Alternative Medicine. A spokesman for the heart institute -- the main sponsor -- declined requests for comment.

The $30 million study, with 1,500 participants so far in more than 100 sites around the country, is testing high doses of vitamin and mineral supplements and chelation (pronounced "kee-LAY'shun"). Though used for lead poisoning, it has not been proved safe or effective for heart disease.

Chelation involves periodic infusions of a drug -- in this case, disodium EDTA, a different substance than what is used to treat lead poisoning. Proponents claim that the drug can flush out calcium that has built up in artery walls. Stiff or clogged arteries can lead to heart problems.

Its use has been highly controversial, and the American Heart Association and other groups have spoken out against it. EDTA carries a risk of kidney failure, bone marrow problems, shock, low blood pressure, convulsions, heart rhythm problems, allergic reactions and breathing troubles, the heart association's Web site says.

The American College of Cardiology is listed in study documents as one of the groups associated with the research. However, the college's chief executive officer, Dr. Jack Lewin, called chelation "an unscientific remedy" and said: "I have always felt this was unscientific experimentation" to do this study at all.

Researchers suspended enrollment last August, after the government started investigating a complaint by a group of scientists that people in the study were not being fully informed of risks and adequately protected. For example, the consent form did not tell participants that dozens of people have died from chelation.

Study leaders made changes to the consent form last year, including that death is "a rare complication." Federal officials recommended other changes to the form or information given to patients, and asked for a response from the three institutions leading the study: Mount Sinai Medical Center in Miami Beach, Fla.; the University of Miami; and Duke University Medical Center in Durham, N.C.

The study's chief researcher, Dr. Gervasio Lamas, recently left the University of Miami to return to Mount Sinai. His office has referred requests for comment to the federal heart institute, the main sponsor. An institute spokeswoman said several months ago that most study sites had allowed enrollment to resume.

The research protection agency's letter is "a remarkably damning statement" that verifies many of the critics' complaints, said Dr. Kimball Atwood, an anesthesiologist in suburban Boston and an assistant clinical professor at Tufts University.

He said the trial should be stopped, and should never have been approved in the first place because not enough previous work had been done to suggest that the treatment is safe or effective.

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