(Associated Press) -- Heart attack survivors are again being enrolled in a controversial federal study of an alternative treatment while the government investigates whether they were told enough about possible health risks.

The $30 million study, with 1,500 participants so far, is one of the largest alternative medicine experiments ever launched. It tests high doses of vitamin and mineral supplements and chelation, a treatment used for lead poisoning that has not been proved safe or effective for heart disease.

Researchers suspended enrollment last August, when the federal Office of Human Research Protections began a probe into whether the people in the study were being fully informed of risks and adequately protected.

Chelation (pronounced kee-LAY-shun) involves intravenous doses of a drug, in this case disodium EDTA. Proponents claim it can flush out calcium that has built up in artery walls. Stiff or clogged arteries can lead to heart problems. There already are several conventional treatments for heart disease, including medicines, surgery and artery-clearing angioplasty.

When the study began in 2002, it aimed to enroll 2,400 people at more than 100 sites in the United States and Canada. But recruitment has lagged, and study leaders now hope to enroll at least 1,700.

Last year, several scientists published an article criticizing the experiment and complained to the federal research protection office. The study's consent form does not tell participants that others have died from chelation, the critics say. More than half of the doctors running the study make money by selling chelation treatments - a conflict of interest, the scientists contend.

The federal agency determined there was merit to the complaint and opened a probe.

"That investigation is still ongoing," said spokeswoman Pat El-Hinnawy.

The study leader, Dr. Gervasio Lamas, and other researchers voluntarily suspended enrollment last fall. Most sites resumed in January, said a spokeswoman for the National Heart, Lung and Blood Institute, which is sponsoring the study with the National Center for Complementary and Alternative Medicine.

Lamas recently left the University of Miami's Miller School of Medicine and is now with Mount Sinai Medical Center in Miami Beach, Fla.

A heart institute spokeswoman, Susan Dambrauskas, wrote in an e-mail that the institute would not comment while the federal probe is ongoing.

Chelation has been highly controversial, and the American Heart Association and other groups have spoken out against it. "EDTA isn't totally safe," and carries a risk of kidney failure, bone marrow problems, shock, low blood pressure, convulsions, heart rhythm problems, allergic reactions and breathing troubles, the association's Web site says.

The federal Food and Drug Administration, two leading doctor groups and others have called chelation experimental and of unknown value or risk for heart disease patients.

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