An autopsy failed to determine what caused the death of a 32-year-old Argentine athlete during the swimming leg of the New York City Triathlon on Sunday. The city medical examiner's office said Monday more tests are needed for Esteban Neira, who died Sunday during the 1,500-meter swim. Several of the 3,000 competitors swimming in the Hudson River complained of jellyfish stings, which can cause allergic reactions and in rare cases can be fatal.