The Athletics Integrity Unit said in a statement Wednesday that it banned 20 athletes from competing in the Tokyo Olympics due to anti-doping rules.
The AIU's list -- which did not identify the athletes by name -- includes 10 from Nigeria, three from Belarus, one from Ethiopia, two from Kenya, one from Morocco and three from Ukraine. Three athletes from Bahrain were also closely monitored, but all three were permitted to compete.
These seven nations -- called "Category A" countries -- are categorized as "being the highest doping risk to the sport under the World Athletics anti-doping rules," according to the AIU, and their athletes were subject to increased anti-doping measures. These included three no-notice, out-of-competition tests (blood and urine) conducted no less than three weeks apart in the 10 months running up to the Games.
It is unclear whether the athletes banned had already traveled to Tokyo to compete. Kenya had replaced two of its athletes before submitting its final entry list to World Athletics.
The decision appears to have affected Nigeria's track and field hopes after it submitted just two athletes for the men's 4x100m and two for the mixed 4x400m, according to the event entry list provided by the International Olympic Committee.
The Athletics Federation of Nigeria (AFN) released a statement to clarify the situation: "The AFN bears responsibility for any lapses that may have occurred during the process and reassures Nigerians that our performances will not be negatively impacted.
"All our athletes resident in Nigeria and who qualified for the Olympic Games completed the three mandatory tests. Most of our top athletes resident in the USA also completed their tests.
"However, a few athletes in the American collegiate system were tested, but those tests were deemed not to have complied with Wada sample collection and analysis standards.
"It must be noted that no Nigerian athlete tested positive to prohibited substances.
"The AFN has taken proactive steps to avoid future occurrences by appointing Professor Ken Anugweje as the head of the Medical and Anti-Doping Commission of the Federation."
The statement was released following criticism from Nigerian athletes. Track and field athlete Blessing Okagbare, who is eligible to compete at the Games, tweeted: "I have said it before and I will say it again.
"If you do not know the sport, not passionate about it/Us [the athletes], then you have no business there as an administrator. The sport system in Nigeria is so flawed and we athletes, are always at the receiving end of the damages.
"They were busy fighting over power, excercising their pride over puma contract/ kits forgetting their major responsibility 'THE ATHLETES'
"It's sad that this cycle keeps repeating its self and some people will come out to say I am arrogant for speaking my truth. It is my CAREER."
Chair of the AIU board David Howman said: "National Federations must play their part in supporting anti-doping efforts. The eligibility rules for athletes from 'Category A' countries are very clear and compliance is essential for cementing the required long-term changes and ensuring a level playing field for clean athletes.
"I must underline that there have been significant improvements in anti-doping efforts in most 'Category A' countries thanks to this rule.
"It is clear that the relevant National Federations in conjunction with their NADOs [National Anti-Doping Organisations] have started to take their testing responsibilities seriously, and I thank them for their efforts, but there remains a long way to go in some circumstances."