(Reuters) – The U.S. Food and Drugs Administration said on Wednesday it was expanding its investigation into impurities in U.S. generic drugs beyond a class of medicines for high blood pressure known as angiotensin II receptor blockers (ARBs).

The agency did not specify which types of additional drugs it was looking at, but said it was testing samples of other medicines with similar manufacturing processes to those in which concerning impurities have been discovered.

Since last summer, the FDA has been conducting a major investigation to address the presence of certain impurities that could cause cancer in ARBs such as valsartan, the generic of Novartis’ Diovan. Some generic ARBs, such as valsartan and irbesartan, have been recalled.

The recalls began last year after regulators said ingredients used by Chinese manufacturer Zhejiang Huahai Pharmaceuticals Co to produce valsartan contained potentially cancer-causing impurities. Since then, generic drugmakers such as Mylan NV, Teva Pharmaceutical Industries and Novartis’ Sandoz have recalled products containing the tainted ingredients.

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It is difficult to know which classes of drugs the FDA is testing in its expanded investigation because companies keep their manufacturing processes secret, according to David Light, chief executive of Valisure, an online pharmacy that chemically tests the medication its sells for flaws.

“Could this be happening in a variety of other drugs? Absolutely,” Light said.

(Reporting by Michael Erman in New York and Manas Mishra in Bengaluru; Editing by Arun Koyyur and Bill Berkrot)