Antidoping scientists who conducted the drug testing at this summer’s Tour de France used their best weapon: the element of surprise.

Unbeknown to the riders competing in the three-week race, those scientists used a test so new that it has yet to be validated by the World Anti-Doping Agency. The test detects a chemical called a plasticizer, which is commonly found in blood-storage bags and leaches into the blood stored in them. High levels of that chemical could suggest that a rider had undergone a banned blood transfusion to increase endurance.

The test caught at least one rider, the three-time Tour winner Alberto Contador, off guard. Contador, a person briefed on the results said, had high levels of the plasticizer in his blood on at least one day of the Tour. That person wanted to remain anonymous because he had agreed to keep the information confidential while Contador’s investigation was continuing.

Scientists are probably using the new test on other riders’ blood samples. Antidoping experts would most likely want to compare Contador’s levels of the plasticizer with those of his peers before moving forward with any charges, said two people with knowledge of the case.