The 31-year-old shotstopper was suspended on December 23 pending an internal investigation, but the Northamptonshire club have only just announced the reason for the ban
He tested positive for a prohibited substance, and faces a Football Association investigation.
A club statement read: "On December 23, 2004 we announced that Billy Turley had been suspended pending the outcome of a disciplinary investigation.
"We are now in a position to reveal that the disciplinary procedure concerned a positive result for a prohibited substance following a drugs test."