"Yes, I tested positive for modafinil at the (U.S.) nationals," Harrison said in a telephone interview from his home here.
"I was notified that they (the U.S. Anti-Doping Agency) had tested my A sample and that they would test my B sample, which they probably have by now."
Harrison's modafinil positive is the latest in a series of positive tests which have rocked track and field on both sides of the Atlantic and could leave next year's Athens Olympics missing a host of banned big-name athletes.
It also follows the discovery of the previously undetectable designer steroid tetrahydrogestrinone THG.
Four U.S. athletes have tested positive for the steroid, American officials have said, as has Britain's European 100 metres champion Dwain Chambers, who said through his lawyer that if he did take the drug it was unwittingly.
"I did have modafinil in my system," Harrison said. "However, I am not in the least advocating the taking of any illegal substances because I strongly believe in fair play."
Harrison said he was given the substance by a coach in California. But the athlete stressed he had never been given an illegal substance by his current coach, Trevor Graham.
"He (the coach in California) had given me this pill and I had taken it. He told me it was not a steroid and that it would just keep you 'up' so you wouldn't be so fatigued," said Harrison.
"He emphasised that it was not on the banned substance list and assured me that it was not an illegal substance."
Modafinil is the same stimulant that double world sprint champion Kelli White of the U.S. tested positive for at the world championships in Paris in August.
SLEEPING DISORDER
While not specified by name on the banned list, modafinil is covered under the stimulants category of "related substances", the International Association of Athletics Federations (IAAF) has said.
White has said she was prescribed the drug for a sleeping disorder. Her case is now under review by the U.S. Anti-Doping Agency and if she is found to have committed a doping offence, she could lose her Paris gold medals.
Harrison's positive, if confirmed by the testing of his B urine sample, could also put in jeopardy the 4x400 metres relay gold medal of the U.S. at the world championships, as he ran the opening leg.
He was also a member of the victorious 2000 Olympics 4x400 metres relay team, who could have their gold medal taken away because Jerome Young ran in the early rounds after failing a 1999 doping test.
The International Olympic Committee (IOC) has called for an investigation into why Young was allowed to run.
World indoor 1,500 metres record holder Regina Jacobs has also tested positive for THG, The Washington Post and The New York Times have reported, and American and British newspapers have also said U.S. hammer thrower John McEwen has tested positive for a banned substance.
Neither athlete could be contacted by Reuters for comment.
