The governing body of men's tennis says it will widen its investigation into the sport's drugs problem following the Greg Rusedski controversy.
Eight players have now escaped drugs bans in the last year because the ATP admitted its trainers could have been distributing contaminated substances.
"The ruling underscores the problem that we identified last year and still face today," said ATP chief Mark Miles.
"Low-level trace occurrences appear to be continuing."
The ATP said it stopped its trainers handing out supplements in May 2003, but Rusedski tested positive two months after that date.
And Miles revealed that the ATP has this year uncovered 16 further samples which showed elevated levels of nandrolone, but which did not exceed the legal limit.
In a statement released after Rusedski was cleared, the ATP said it would "re-double efforts to identify the cause of these test results".
To this end, the governing body has recruited two new experts to help with the ongoing investigation into the problem.
"We are determined to provide them with all necessary resources to accomplish the goal of identifying the cause of the problem that was essentially non-existent prior to the summer of 2002," said Miles.