Jones flew home from Switzerland on Friday

"I was shocked when I was informed about the positive 'A' sample," the American, 30, said in a statement.

Reports suggest that erythropoietin or EPO, a blood-boosting drug, was found in a sample given by Jones in June.

Jones is waiting to learn if her second sample is also positive and wants the process completed as soon as possible.

She faces a two-year ban if the second test comes back positive.

"I have requested that the testing of my 'B' sample be expedited and done as soon as possible," she added.

"I would also like to note that only my lawyers have the authority to speak on my behalf in this matter, and I will have no further comment until the results of the 'B' sample are released."

Jones has repeatedly denied taking performance-enhancing substances and had never previously failed a doping test.

She had been under scrutiny by the US Anti-Doping Agency (USADA) in connection with the Balco laboratory doping scandal but has never been charged with a doping offence.

Her statement did not name the substance for which her 'A' sample was positive.

A statement from her attorney, Howard Jacobs, said the adverse finding was on her June 23 'A' sample.

Jacobs has represented a number of US athletes in anti-doping cases, including Tim Montgomery, a former partner of Jones.

Montgomery was banned from the sport for two years based on evidence from the Balco scandal and his 2002 world record time of 9.78 seconds for the 100 metres was expunged.

Jones initially cited "personal reasons" for her surprise withdrawal from the recent Golden League meeting in Zurich.