L'Equipe newspaper reported Tuesday on its Web site that prosecutors opened the case after "the discovery of several suspicious syringes in a container given by organizers to all the teams in order to collect the medical waste."
Tour de France winner Alberto Contador and third-place Lance Armstrong rode with Astana on this year's Tour. The Spaniard is still under contract with the Kazakh-funded team while the Texan left to launch his own squad, RadioShack.
According to L'Equipe, the syringes will be analyzed by a forensic lab called Toxlab, which is responsible for determining their content.
Gilbert Pepin, a doctor at the lab, was not available for comment and did not immediately respond to an e-mail from The Associated Press.
While there were no positive tests at this year's Tour, authorities did seize drugs that the French anti-doping agency's scientific adviser said raised questions.
The drugs _ which include treatments for diabetes, high blood pressure and convulsions suffered by manic-depressives _ are not banned but are "incongruous" in top-level athletes, said Professor Michel Rieu last week.
Rieu declined to say which teams the substances had been taken from, saying that it was part of a judicial investigation.