The head of the World Anti-Doping Agency has said he believes hundred of athletes at the Olympics could be drug cheats.
The Director General of WADA David Howman made the comment as details of a positive test were revealed. Morocco's Mariem Selsouli, one of the favourites for women's 1500 metres gold medal, is out of the Games.
Mariem Selsouli gave a sample which contained a banned diuretic shortly after running the fastest time this year over 1500 metres in Paris earlier this month.
She has become one of more than a hundred athletes worldwide who have been caught in the last six months.
The anti-doping operation at this Olympics will be the most rigorous in history.
There are new ways to detect banned substances like human growth hormone and synthetic testosterone which have previously dipped beneath the radar.
But despite the advances, the Director General of the World Anti-Doping Agency has expressed caution.
David Howman feared as many as one in ten athletes at the Games could be cheating the system. And he said the sophisticated dopers are difficult to trap.
WADA said intelligence gathering had become key in identifying potential cheats and that individual athletes have been targeted.
Samples taken in London will be kept for the next eight years. The message to those who want to use banned products was they would never be able to rest at ease.
Although the International Olympic Committee has been criticised for not re-testing the batch from the 2004 Games in Athens until recently.
a sample: sampel air seni
diuretic: obat untuk meningkatkan aliran air seni
rigorous: sangat ketat
to detect: mendeteksi
dipped beneath the radar: luput dari perhatian
sophisticated: paling rumit, canggih
potential cheats: atlet yang dicurigai menggunakan obat bius
to rest at ease: beristirahat dengan tenang