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.
"He's a different guy, but I never thought he would risk his career. Gotta help him bounce back now. Can't turn my back on him. Disappointed yet concerned about the man. Still love him, though," a Broncos player, who wished to remain anonymous, said in a text to ESPN.
If Miller's suspension stands, the Broncos could go after 1/16th of his signing bonus, which would amount to $860,812.
At the very least, Denver will be dealing with another significant issue during training camp. Last week, the team suspended director of player personnel Matt Russell indefinitely without pay and director of pro personnel Tom Heckert for one month without pay for their recent drunken-driving arrests.
The Broncos report to training camp Wednesday. Miller will be able to participate in all of the workouts but he'd have to leave the team after its final preseason game on Aug. 29 if a suspension is upheld and he wouldn't be able to return to the team until Sept. 30.
If Miller is suspended in Week 1, the Broncos would be facing the defending champion Ravens without both the star linebacker and Elvis Dumervil, who was released by Denver this offseason after a deadline fax fiasco and later signed with Baltimore.
The Broncos visit the New York Giants in Week 2 before home games against the Oakland Raiders and Philadelphia Eagles. If Miller is suspended for the first four games of the season, he would make his debut in Week 5 when the Broncos visit the Dallas Cowboys.
Miller, 24, had 18.5 sacks last season for the Broncos. According to ESPN Stats & Information, since Miller entered the league in 2011 -- he was named the AP's Defensive Rookie of the Year -- only Jared Allen (34), Aldon Smith (33.5) and DeMarcus Ware (31) have more sacks than Miller (30).
Miller set the bar high for the Broncos this offseason when he pulled a Joe Namath in March and guaranteed a Super Bowl title. He dedicated the season to his 6-year-old cousin who had just emerged from a coma after a car accident in West Texas that also injured his mother and 8-year-old brother.
"Guaranteeing it wasn't fueled out of being cocky or anything like that,'' Miller said at the time. "I actually feel like what else is there? What else is there for the type of team that we have and the type of guys that we've got in the locker room?"
Information from The Associated Press was used in this report.