The World Anti-Doping Agency has slammed Rio 2016 organisers for "serious failings" in drug testing during the Olympic Games.
A 55-page report from the WADA Independent Observers claims that on some days during the event close to 50% of scheduled drug tests were not carried out because athletes could not be found.
The report reveals that there were few or no blood tests taken in high-risk sports, including weightlifting, while WADA found it "surprising" that there was no out-of-competition testing in the football event.
Rio 2016 organisers were accused of failing to give chaperones - the people responsible for notifying athletes of drug testing - the right training and information.
The report read: "Chaperones were often provided with little or no whereabouts information for athletes targeted for out-of-competition testing in the Athletes' Village, and therefore, the majority of times had to resort to asking team officials and/or athletes from the same team where the athletes they were looking for were located.
"Providing the names of the athletes they were seeking was, at best, highly inefficient and obviously compromised the 'no notice' nature of the testing. In addition, when initial attempts to find an athlete in his or her room were unsuccessful, chaperones often lacked the training and/or the confidence to follow up with further enquiries and effort to find the athlete in other locations in the Village, such as the dining hall.
"Ultimately, many athletes targeted for testing in the Athletes' Village simply could not be found and the mission had to be aborted. On some days, up to 50% of planned target tests were aborted in this way."
The report added that without the dedication of the doping control staff, "the anti-doping program would have almost certainly collapsed".
It has been revealed that around 500 planned drug tests were not carried out during the Games.