According to the well-informed Ciclismo Internacional website, Najar was told his A-sample was positive at the end of the week and he immediately left the Argentinian national team with whom he was preparing for the Pan-American Games.
According to Ciclismo Internacional, Gastón Javier, who played a key part in Najar's win at the Vuelta a San Juan, also tested positive for testosterone.
The UCI has yet to confirm the positive cases but reportedly will do so on Monday.
His win surprised many as he distanced former doper Oscar Sevilla, Tiesj Benoot (Lotto Soudal) and Dayer Quintana (Movistar). Najar went on to win the overall classification 51 seconds ahead of Sevilla.
Najar has not raced on the road at international level since the Vuelta a San Juan in late January.
In February, a post from the official Twitter account for the Vuelta a San Juan claimed that there was a positive anti-doping result from the January race and that more information would follow. However, the post was quickly deleted and the organisation later said that a hacked Twitter account was responsible for a message that went out over social media.
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However, one month later, Ignacio Maldonado, of the Uruguay National Team, tested positive for CERA in a control carried out after the first stage of the Vuelta a San Juan.
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