River Plate have announced that their players, Camilo Mayada and Lucas Martinez Quarta, tested positive for a banned substance during the Copa Libertadores.
The Argentinian giants were notified this week by CONMEBOL, the South American Football Confederation, that hydrochlorothiazide, a diuretic, had been found on the two players' samples.
Uruguay international midfielder Mayada failed a doping test on May 18 after a 3-2 win over Peruvian side Melgar while the test on Argentinian defender Martinez Quarta was conducted after River's 1-1 draw against Ecuador outfit Emelec eight days earlier.
A statement from River said: "Considering the situation of Camilo Mayada and the case of Lucas Martinez Quarta, the club is working on understanding the situation."
River have requested B samples and have begun their own investigation as they believe both cases are a result of contaminated sports supplements.
According to La Nacion, there may be more cases with other River players also having had the same supplement as that of the Mayada and Martinez.
"In view of these two cases, it reinforces with vehemence the assumption of a contamination regarding some of the products usually used as supplements," added the club statement.
Martinez, 21, has been provisionally suspended by the Disciplinary Commission of CONMEBOL.
Donato Villani, the vice president of CONMEBOL's medical commission, said he was surprised by the results.
"It's obvious that there must have been a contamination with a diuretic with a supplement that they are using," he told Ole newspaper. "The diuretic has been prohibited for some time. It's surprising that this has happened at River."
River resume the Copa Libertadores on July 4 in a round-of-16 encounter against Guarani in Asuncion on July 4.