Lokomotiv Moscow youth player Innokenty Samokhvalov has died, aged 22, after suspected heart failure during a solo training session on Monday.
With the club's players quarantined during the coronavirus pandemic, Samokhvalov was training on his own.
"On April 20, defender Innokenty Samokhvalov died," the club said. "The footballer became ill during an individual training session.
"Details of the tragedy are being clarified."
Samokhvalov arrived at Lokomotiv in 2015, but had not made any appearances in the Russian Premier League. He was playing for Lokomotiv's reserve team Kazanka in Russia's third tier.
"It's horrible," Kazanka coach Alexander Grishin told Russian news agency RIA Novos. "The doctors have told us the cause of death was heart failure.
"If the doctors allowed him to train, then we must assume he did not have any problems."
Russia is in lockdown until April 30, while competitive football has been postponed until May 31.
"Samokhvalov left a wife and a son," added the club's statement. "FC Lokomotiv is shocked by what happened.
"This is a huge sorrow for our family. We express our condolences to his family and friends."