The Food and Drug Administration is investigating whether the nation’s largest operator of dialysis centers violated federal regulations by failing to inform customers of a potentially lethal risk connected to one of its products, an F.D.A. official said.

The company, Fresenius Medical Care, treats more than a third of the estimated 400,000 Americans receiving dialysis. It also is the leading supplier of dialysis machines and disposable products, which are used by many clinics in addition to its own.

Last November, Fresenius’s medical office sent an internal memo to doctors practicing in the company’s dialysis centers, warning them that failure to properly use one of the company’s products appeared to be contributing to a sharp increase in the risk of patients dying suddenly from cardiac arrest.

“In light of these troubling findings,” the memo said, doctors should take corrective action. “This issue needs to be addressed urgently,” the memo added later.