Triple Olympic sprint champion Marion Jones has failed an initial doping test, a source familiar with the test told Reuters.
The positive test of Jones's A sample occurred at the June U.S. championships in Indianapolis, the source, who did not wish to be identified, said.
Under anti-doping protocols, an athlete is not considered to have committed a doping offence until his or her B sample has tested positive and the athlete has been afforded an arbitration hearing.
Jones would face a minimum of a two-year ban if it is determined she has committed a doping offence.
Neither the U.S. Anti-Doping Agency (USADA) nor the U.S. Olympic Committee (USOC) would comment.
Jones's general counsel also would not discuss the matter.
"It is unfortunate that the integrity and the confidentiality of the testing process may have been breached, but Marion Jones has always been clear, she has never taken performance-enhancing substances, not now, not ever," Rich Nichols said in a statement to Reuters.
"She is always prepared to do whatever it takes to demonstrate that she has never taken performance-enhancing substances."
He would not elaborate.
The 30-year-old Jones has been under investigation by USADA in connection with the BALCO laboratory doping scandal.
She withdrew suddenly from the Golden League meeting in Zurich on Friday and flew back to the United States for personal reasons, organisers said.
Jones is the third high-profile American athlete to fail a doping test this year.
Olympic 100m champion Justin Gatlin tested positive for testosterone at a relay meeting in April and cyclist Floyd Landis failed a test for testosterone at the Tour de France.
Jones's former coach, Trevor Graham, is also under investigation by USADA and the International Association of Athletics Federations (IAAF). He is the current coach of Gatlin.
After two years of poor performances following the birth of her son in 2003, Jones made a triumphant return this year.
She won the U.S. 100 metres championship and several races in Europe.
Jones won gold in the 100m, 200m and 4x400m relay in the 2000 Olympic Games in Sydney.