At the center of the new investigation is the McLeod Addictive Disease Center, a company whose clinics statewide offer the synthetic substance, typically used to help heroin addicts during withdrawal, the Charlotte (N.C.) Observer said Sunday.

The state Department of Health and Human Services inquiry comes after the Charlotte company was linked to the deaths of 13 of its clinics' patients this year, the newspaper said.

Health officials have said the probable cause of death among those individuals, along with three other patients in 2006, was the use of methadone with other drugs or methadone toxicity.

McLeod President Eugene Hall has denied any wrongdoing at his company's clinics, saying all of the sites operate based under both state and federal guidelines.

"I've welcomed (state officials) down to look at our program," he told the Observer. "If we're not doing it right, then nobody's doing it right."