Impurities that triggered the international recall of a life-saving HIV medicine this year were caused by the failure of Roche, the Swiss pharmaceutical group, to understand the manufacturing process, says a regulator’s report released on Friday.

For more than five years, employees did not clean a “hold tank” that stored the starting material for Viracept (nelfinavir), and then allowed the ingredients to mix with ethanol used to clean the vat, creating a toxic compound later discovered in the drug distributed to patients.