Sylvester Stallone is facing possible banned substance charges for reportedly bringing illegal bodybuilding supplements into Australia. The bulked-up actor was detained by customs officials when a routine X-ray set off red flags during his arrival in Sydney on Friday night, causing him to miss the opening of Rocky Balboa. Contraband items were seized from Stallone, and although customs officials have not revealed any details, the seized materials are believed to be human growth hormone doping drugs used in bodybuilding. "I do know [why I was stopped] but I can't tell you," Stallone told a Sydney paper on the weekend. "To [customs officials] it's major, but it's really minor stuff. I just made a mistake... I misunderstood a few things and we are going through the process." Stallone has not spoken to the media yesterday's raid on his hotel and jet, which allegedly resulted in additional evidence being seized, and it has been widely reported that when officials arrived at his hotel to issue a summons, Stallone and whoever he is traveling with were seen tossing something from the windows of their room. "The investigation is ongoing and we are aware of Mr. Stallone's travel movements," said a customs spokesperson. "The material has to be identified and we have to look at all the options and the evidence before deciding what action to take."