Federal prosecutors are investigating whether Valeant Pharmaceuticals International Inc. defrauded insurers by shrouding its ties to a mail-order pharmacy that boosted sales of its drugs, people familiar with the matter said.

The lawyers, in the U.S. attorney’s office in Manhattan, are pursuing an unusual legal theory, previously unreported, that Valeant and a closely linked mail-order-pharmacy, Philidor Rx Services LLC, allegedly defrauded insurers by hiding their close relationship, the people familiar with the matter said....