Russia winger Denis Cheryshev, one of the standout performers at the World Cup, is under investigation for an alleged doping violation, local media reported Wednesday.
"This case is currently being investigated by the Spanish Anti-Doping agency," the Russian daily Sport Express quoted a World Anti-Doping Agency press spokesperson as saying.
"They will inform WADA about the results of their investigation."
The 27-year-old came under suspicion after his father said in an interview that the footballer had received growth hormone treatment before the World Cup.
The taking of a growth hormone without a therapeutic use exemption can result in a four-year ban.
Cheryshev's father later said he was misquoted by the journalist.
The Villarreal midfielder, on-loan to Valencia this season, meanwhile denied any wrongdoing.
"I think we'd better leave this case in the hands of medics, who have done everything right," he told Sport Express.
"For my part everything was honest and I think there will be no problems."
Cheryshev scored four goals as hosts Russia reached the World Cup quarter-finals.