The United States Anti-doping Agency issued a statement saying Crawford had been handed a two-year suspension because he had failed to notify drug testers of his whereabouts during the last 18 months.
But his agent told Reuters in an email on Friday that Crawford, who voluntarily gave his silver medal from the 2008 Beijing Olympics to a rival competitor he felt had been wronged, had already announced his retirement from athletics after failing to make the US team for last year's London Olympics.
"Crawford retired after the trials last year," Crawford's representative Kimberly Holland said. "He's 35 years of age, and has moved on.
"Unfortunately, his announcement wasn't enough, as he had to complete retirement paper work to be removed from the testing pool."
Athletes are required to provide details of their whereabouts to drug-testers for a certain period of each day so they can be located for out-of-competition tests.
USADA said Crawford failed to file his whereabouts and accrued three "Whereabouts Failures", which constituted a rule violation.
USADA said Crawford's third whereabouts failure occurred in November last year and his two-year ineligibility began on Wednesday, when the ban was officially imposed.
Crawford also won a silver medal in the 4x100 relay at Athens and the individual 200m at Beijing, won by Usain Bolt.
He crossed the line fourth in Beijing but was promoted to second after Churandy Martina of the Netherlands Antilles and American Wallace Spearmon were disqualified for running out of their lanes.
Crawford later gave his silver medal to Martina, saying he believed he deserved it, although Martina lost an appeal to be officially reinstated.