Brazilian police searched on Monday for the driver of a taxi in which U.S. gold medalist swimmer Ryan Lochte and three teammates were robbed at gunpoint, as authorities stepped up security in Rio de Janeiro after a spate of incidents.

The swimmers were returning to the Athletes' Village in the early hours of Sunday after a party hosted by the French Games delegation when armed men carrying police badges pulled them over. The gunmen ordered them to drop to the ground and demanded their wallets and belongings, Lochte said.

When he refused, one of the men cocked a pistol and placed it against the swimmer's head. He then handed over his wallet.