FedEx and United Parcel Service are being investigated by the Securities and Exchange Commission (SEC) in a probe of prescription medication sales by online pharmacies, the two US package delivery companies said.
The Wall Street Journal, which first reported on the matter, wrote that the probe is part of an expanding crackdown against illegal sales of prescription painkillers.
In a statement, FedEx denied any wrongdoing and vowed to "cooperate with the investigation."
"We do not believe that we have engaged in any illegal activities and will vigorously defend ourselves in any action that may result from the investigation," the Memphis, Tennessee based company said.
UPS, which is based in Atlanta, Georgia, issued a statement saying it too, was cooperating with the probe.
"We have been cooperating with this investigation and are exploring the possibility of resolving this matter," it said.