Pfizer Inc. on Tuesday said it is cooperating with a federal investigation into whether it is improperly marketing Lipitor, a cholesterol-lowering drug that is the world’s top-selling medicine.

“We do not know who initiated or why an investigation was initiated in Brooklyn, but we have been cooperating with that investigation and have seen no merit in any of the claims in that matter,” a spokesman for Pfizer, Andrew B. McCormick, said in an e-mail Tuesday.