Ranbaxy has been stunned by an investigation by US officials who have searched the Indian drugmaker’s US headquarters in New Jersey.

The firm confirmed that its offices in Princeton were searched and noted that “this action has come as a surprise” and “the company is not aware of any wrongdoing.” Ranbaxy added that “it is co-operating fully with the officials,” adding that its operations “continue as normal.”

Charles Caprariello, a spokesman for Ranbaxy’s affiliate in the USA, told Reuters that the officials were from the US Food and Drug Administration “took a variety of documents" and added that “it's been a wide dragnet – a sweep of questions – and we've tried to work with them in answering everything they've asked."

Mr Caprariello continued that “there are a lot more questions than answers right now" and asked whether the investigation was criminal or civil in nature, he said, "It would be premature for anyone to speculate at this time. We're trying to probe to find out more." The agency said it could not "confirm or deny FDA involvement" in the Ranbaxy case, while the Department of Justice said it could not comment on pending investigations.

The US is Ranbaxy’s key market and sales there of simvastatin, its generic form of Merck & Co’s cholesterol Zocor helped the firm post a 167% earnings increase for the fourth quarter to 1.86 billion rupees ($42 million). Ranbaxy recently launched a generic form of the blockbuster cholesterol-lowerer Lipitor (atorvastatin) in Denmark, much to the anger of US drug giant Pfizer.