French police are investigating suspicions that a hairdryer may have been used to tamper with official weather readings to make thousands of dollars in Polymarket bets.

Temperature readings at Paris’s Charles de Gaulle airport have unexpectedly spiked twice in the last month, reaching levels much higher than expected.

On both days, gamblers on Polymarket, the world’s biggest prediction market, appear to have made huge sums by betting on unlikely weather patterns. The site relied on readings from the Charles de Gaulle temperature sensor.

Météo-France, the country’s official weather agency, said it had complained to police after noticing a change to one of its temperature sensors.