The New Zealander at the head of the World Anti-Doping Agency believes hundred of athletes at the London Olympics could be drug cheats.
WADA director-general David Howman made the comments as details of positive tests were revealed on Wednesday.
The International Athletics Federation says that nine track and field athletes have been handed lengthy bans for doping violations in the lead-up to the London Games.
They include two leading Moroccans - women's 1500 metres favourite Mariem Selsouli, who had already served a two-year suspension for doping and now faces a lifetime ban - and leading men's marathon runner Abderrahim Goumri.
The athletics governing body has provisionally banned Selsouli from the Olympics.
More than 71,000 tests have been carried out worldwide by WADA in the six months leading up to 19 July, with 107 athletes sanctioned.
All the athletes were caught with the aid of the Athlete Biological Passport programme, which is being used for the first time in London.
David Howman told the BBC the agency is determined to crack down on any Olympic drug-taking athletes and challenge is always to keep ahead of those who seek new methods to beat testing.
"Often it's difficult, but that's where you've got to try and get other evidence from other sources and we really do need to be on high alert when it comes to that sophisticated area.
"It's not just WADA, it's the IOC who are running the programme here, it's the laboratory, it's everybody who is concerned have to be incentivised to say, 'Let's catch these guys and make it better for the clean athlete'".
Anti-doping investigations at the London Games will be the most rigourous in history.