Bystanders were able to rescue the girl's mother after her car fell into the Tarn River near the village of Mirepoix-sur-Tarn, regional prosecutor Dominique Alzeari told reporters.
Several hours later, Mirepoix-sur-Tarn Mayor Eric Oget said on BFM television that a truck driver whose vehicle fell into the river was also found dead, and rescuers were working to extract his body.
Five people were injured in the collapse, including two firefighters at the scene, according to the prosecutor.
An investigation into the cause of the collapse has been opened.
The bridge had been renovated in 2003 and its last safety check was in 2017. The mayor said it was in good condition, and suggested a truck might have violated the bridge's weight limit.
French Prime Minister Edouard Philippe, on a visit to Senegal, deplored the "tragic accident."
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