World and Olympic 100 metres champion Justin Gatlin has admitted failing a drugs test in April.
Gatlin shares the world record with Asafa Powell at 9.77 seconds
The American, 24, was told the news by the United States Anti-Doping Agency.
He said: "I have been informed that after a relay race I ran in Kansas City on 22 April, I tested positive for 'testosterone or its precursors'.
"I cannot account for these results, because I have never knowingly used any banned substance or authorised anyone to administer such a substance to me."
Gatlin, who also shares the world record with Asafa Powell, added: "Since learning of the positive test, I have been doing everything in my power to find out what caused this to happen.
It is simply not consistent with either my character or my confidence in my God-given athletic ability to cheat in any way
"I have been and will continue to cooperate fully with Usada as it moves forward with the process it has initiated and hope that when all the facts are revealed it will be determined that I have done nothing wrong."
Gatlin tested positive for an amphetamine at the 2001 US Junior Championships.
A subsequent ban was overturned after it was discovered the illegal substances were part of the medication he had been taking for a decade to combat attention deficit disorder.
But the International Association of Athletics Federerations (IAAF) made it clear at the time that a further doping violation would signal a life ban for Gatlin.
In his statement on Saturday, Gatlin said: "That experience made me even more vigilant to make certain that I do not come into contact with any banned substance for any reason whatsoever, because any additional anti-doping rule offence could mean a lifetime ban from the sport that I love.
USA Track & Field is gravely concerned that Justin Gatlin has tested positive for banned substances
USATF chief executive
"Since the positive test at the University of Tennessee, I have been involved with efforts to educate people about the dangers of using drugs and would never do anything to disappoint my fans and supporters.
"It is simply not consistent with either my character or my confidence in my God-given athletic ability to cheat in any way."
The IAAF confirmed on Sunday that the New Yorker would be handed a lifetime ban if his positive test is confirmed by the US authorities.
Gatlin faces a hearing before the Usada, who cited him for the doping offence in mid-June.
Athletics had been waiting all year to see a showdown between world record holders Gatlin and Powell, but Gatlin, blaming injuries, has not raced since June and missed Friday's London Grand Prix.
Gatlin came into the sport from an American Football background and only turned professional in 2003.
He won the Olympic 100m title in Athens in 2004 and followed that up by taking the sprint double of 100m and 200m at last year's World Championships in Helsinki.
Gatlin then matched Powell's world record time of 9.77 seconds in Qatar in May of this year, one month after his positive test in Kansas City.
In a statement, USATF chief executive Craig Masback said: "USA Track & Field is gravely concerned that Justin Gatlin has tested positive for banned substances."
He added: "We hope Justin has not committed a doping offence, and we await the completion of the adjudication process."