The Court of Arbitration for Sport said it held a hearing Monday on the previously confidential case of Brenton Rickard, who tested positive years after the London Games in a reanalysis of his sample. Rickard won a bronze medal in the 4x100-meter medley relay.
The case was revealed last week by Australian media.
Rickard, a three-time Olympic medalist, swam in the heats for Australia. The United States won gold with a team that included Michael Phelps.
CAS said the case between Rickard and the International Olympic Committee was heard by video link. The court did not set a timeline for a verdict.
Samples from the 2012 Olympics have been allowed to be stored for eight years and re-tested when laboratories develop better methods.
Rickard tested positive for furosemide, a diuretic and masking agent that can hide the use of other banned substances. No trace was detected in Rickard's urine sample at the time.
The IOC declined to comment on the case, citing legal reasons. It had not previously announced details of the case, which is one of about 60 positive tests found in the London reanalysis program. Most were in weightlifting and track and field, involving athletes from eastern Europe.
Rickard's sample reportedly had a tiny amount of the banned substance, which Australian media believe came from a contaminated over-the-counter medication.
Strict liability rules in doping mean athletes face disqualification and a ban if they cannot detail exactly how a banned substance became present in their system.
If Australia is disqualified, the fourth-place British team could be upgraded to get bronze medals.
Rickard, who is now 37, won two silver medals at the 2008 Beijing Olympics, one in the 4x100 medley relay and another in the 200 breaststroke.
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