Denver Broncos Pro Bowl linebacker Von Miller is facing a four-game suspension for violating the NFL's substance-abuse policy, according to multiple sources.

The case currently is in the appeals process, so it's uncertain whether the suspension ultimately will stand. No date for Miller's appeal has been set yet, a league source said.

Miller addressed the situation in a tweet Monday afternoon.

Seeing reports abt 4 game susp. I know I did nothing wrong. I'm sure this'll be resolved fairly. Disapp. Broncos have 2 open camp like this. — Von Miller (@MillerLite40) July 22, 2013

The league has a strict confidentiality clause on such cases. If the case works the way it's supposed to, the NFL would only comment if the appeal fails.

NFL Players Association spokesman George Atallah said in a tweet that the union is working closely with Miller on his appeal.

The NFLPA is working closely with @MillerLite40 to appeal his suspension. We are all very disappointed that confidentiality was breached. — George Atallah (@GeorgeAtallah) July 22, 2013

It is unknown which substances Miller is accused of using. Under NFL policy, Miller would have to test positive for banned substances twice before facing a four-game suspension.

Atallah didn't say what Miller tested for, but did later tweet, "I can also confirm that Von's case does not involve the Steroids and Related Substances Policy."

The Denver Post reported Monday that Miller tested positive for amphetamines and marijuana in 2011, his rookie season.

A league source said the Broncos were aware of an issue with Miller, so executive vice president of football operations John Elway and coach John Fox were not surprised by Monday's news.

Broncos spokesman Patrick Smyth said the team was unable to comment, The Associated Press reported.