World 400m champion Jerome Young has been banned for life after a second positive dope test, says the United States Anti-Doping Agency.
The 28-year-old tested positive for EPO at a meeting in Paris in July.
"He was suspended for life from all sanctioned competition beginning November 3," said a USADA statement.
"Under International Association of Athletics Federations rules, the sanction for a second offence is a lifetime suspension."
Young, who was did not qualify for this year's Olympics, won gold in the 4 x 400m relay team at Sydney in 2000.
But that team - which included Michael Johnson - may now be stripped of those medals because of his involvement.
He competed in Sydney without the world knowing about his positive 1999 test because the American authorities chose not to take any action against him.
It was only three years later that the test result came to public prominence, and the International Olympic Committee ruled that his results for the two years after the positive test - including Sydney - be annulled.