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 told ESPN's Sal Paolantonio on Wednesday that Brown was "still collecting information" on the injury, and that the team would know more in the next few days.

"You know me, it is like waiting for Santa Claus," Gruden said of anticipating Brown's return. "I have been really excited to coach this man. I think he is a great player and he is a great competitor. We need his life in this offense and on this football team. We will just cross our fingers. I will make no speculation whatsoever. He has to get it evaluated. He has to get the information he needs, and when he does, he will be back."

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.