North Carolina (USA): Triple Olympic sprint champion Marion Jones has failed an initial doping test, a source familiar with the test said.
The positive test of Jones's A sample occurred at the June US championships in Indianapolis, the source, who did not wish to be identified, said.
US media reported the blood-boosting drug erythropoietin (EPO) had been found in Jones's urine sample.
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 US Anti-Doping Agency (USADA) nor the US Olympic Committee (USOC) would comment.
A spokesman for the International Association of Athletics Federations (IAAF) said it was unable to comment on individual cases until the result of the B sample was known or if the athlete in question had been suspended from competition.
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.
"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 although she had never failed a doping test.
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 IAAF. He is the current coach of Gatlin.
Jones won gold in the 100m, 200m and 4x400m relay at the 2000 Olympics in Sydney while being coached by Graham.
She also won bronze medals there in the long jump and 4x100m relay.
She has repeatedly denied taking performance-enhancing substances. However, the San Francisco Chronicle has reported her former husband, banned shot putter C J Hunter, told federal investigators he injected Jones with banned substances and saw her inject herself.
Jones also previously was the partner of disgraced former 100 metres world record holder Tim Montgomery, who was banned for the sport for two years in connection with the BALCO scandal although never failing a doping test.
After two years of poor performances following the birth of her son with Montgomery in 2003, she made a triumphant return this year.
She won the US 100 metres championship in 11.10 seconds and was first in several races in Europe.
She has three of the five fastest times in the world this year with a best of 10.91 seconds to rank second overall behind Jamaica's Sherone Simpson.