"The thumb drive was not mailed from Under Armour to PwC. It was mailed between PwC offices," Under Armour spokeswoman Diane Pelkey said in an email.
The breach of payroll data was first reported by Ohio's Dayton Daily News, which last week obtained an internal memo sent by Under Armour to its employees about the incident. The memo said an unencrypted thumb drive was sent by U.S. mail as part of an audit and was lost on or about April 12.
"This was an isolated incident for PwC," a PricewaterhouseCoopers spokeswoman, Caroline Nolan, said Monday.
PwC said it was working with Under Armour to protect the employees whose information was lost and was investigating how its security protections had been breached.