Mullen returned a positive sample to Drostanolone last November and is on a provisional suspension.

Channel Seven reported Mullen had returned a positive B-sample with the Knights half facing a four-year ban by the Australian Sports Anti-Doping Authority.

At 29, his professional rugby league career is all but over.

Mullen's teammates have also been blood-tested with laptops and mobile phones have been seized.

ASADA are believed to be investigating where Mullen sourced the steroid and if other players are involved.

"The role of the people surrounding the athlete and any other issues pertaining to the use of the substance by the athlete - in doing so, we pursue and follow any leads as part of that process," the statement read.

Mullen has played 211 games for the Knights after debuting in 2005.

He had suffered several injuries in recent seasons including a pre-season injury that threatened to sideline him for 2017.

The No.6 was a marquee player and key playmaker.