Five Jamaican athletes have tested positive for a banned substance, the sport's governing body has confirmed.
The athletes - four men and one woman - are not thought to include any of Jamaica's 2008 Olympic medallists.
IAAF spokesman Nick Davies said: "It's very early in the process so we cannot name them or the substance."
An unnamed Jamaica Anti-Doping Commission (Jadco) official had earlier told news agency Reuters: "None of the world stars are involved."
Dr Herb Elliott of Jadco told Radio Jamaica that all five of the athletes had been selected for Jamaica's 46-strong team for next month's World Championships in Berlin.
Davies added: "We have just received the paperwork from Jamaica, it is five - four guys and one girl. These are in-competition tests.
"We now inform the (Jamaican) federation who will then inform the athletes, and then they are asked if they want the B sample tested."
This is probably a step backwards because people start questioning everybody, especially from Jamaica
Jadco's executive director Dr Patrece Charles-Freeman was quoted in the Jamaican media as saying an investigation is now under way.
Jamaica's Prime Minister Bruce Golding has called top officials from Jadco and the Jamaica Amateur Athletics Association to an emergency meeting in Kingston so he can be briefed on the situation.
The Caribbean nation dominated last summer's Olympic sprint races and is hoping for similar success in Germany.
Former world 100m record holder Asafa Powell commented on the news after racing in a heat at the Aviva London Grand Prix on Friday, saying: "It's disappointing, but I have to do what I have to do. I just have to stay that way and stay clean."
Aleen Bailey, competing in the women's 200m event in London, was visibly shocked by the news.
"You never want anybody from your country or your team to be tested positive," she told BBC Sport.
Olympic 100m and 200m champion Usain Bolt, who cruised to victory in the 100m, said: "It's sad for the sport because it was getting on so well.
"This is probably a step backwards because people start questioning everybody, especially from Jamaica.
"I'm not sure who it is yet, but I'm sure it's not me so I'm not really worried. I'm clean, I'm just going forward."