The head of US anti-doping says he fears some athletes might "take advantage" of reduced drug testing due to the coronavirus pandemic.
Travis Tygart, CEO of USADA, said that various agencies around the world have seen significant reductions in testing, while some places have shut down entirely.
Talking to BBC Sport, Tygart said: "I think clean athletes are going to compete clean and are not going to try and take advantage of this situation.
"What we worry about is those who otherwise may not be willing to compete clean and will try everything to get away with it. It does for a period of time at least open that window of opportunity.
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"We know that there are those who are going to try to exploit every opportunity they can. Certainly, it’s a concern and it’s something the world has to take seriously."
Tygart says USADA are continuing with "mission-critical testing" and that he believed the 2020 Tokyo Games, which has since been postponed, could have been the "cleanest Games we've seen".
The President of the World Anti-Doping Agency (Wada), Witold Banka, agreed with Tygart and added that "anti-doping never sleeps".
"Of course, this a difficult situation but we need to do everything to protect the integrity and the unity of anti-doping and make lots of efforts to do so.
"In some cases testing will be temporarily suspended or limited or cancelled for some time but we have other tools, the testing is not only one weapon.
"This is my strong message to everyone who wants to cheat us: we will catch you. It's not a good space and a good situation for cheats, we will do everything to protect and maintain the unity of the system and I'm sure we will return the system to full power after this pandemic and we will be stronger."
Last month, Spanish police took down a doping wholesaler, seizing 1.6 million drug doses.
The investigation began after 100 boxes of growth hormone were sent to the wrong address last July
