Oakland Raiders star receiver Antonio Brown's mysterious foot injury is the result of extreme frostbite caused by a cryotherapy machine, a source confirmed to ESPN on Wednesday.

Brown was not wearing appropriate footwear when he entered the cryotherapy chamber last month in France and burned his feet, leaving them frostbitten, according to the source.

The cause of Brown's injury was first reported by ProFootballTalk.

Brown, 31, has missed a significant portion of his first training camp with the Raiders and has not practiced since July 30, when he left the team to visit a foot specialist.

Raiders coach Jon Gruden said Tuesday that Brown was "gathering information" on the injury but did not say when the seven-time Pro Bowler would return.

A source told ESPN's Adam Schefter this past weekend that the injury is not believed to be long term.

Brown posted photo on Instagram last week that showed the soles of his feet blistering and peeling.