The wife of the male cyclist, shot dead as bullets sprayed into the car of a British family, went to police to report him missing, unaware of the mass killing.
The man, named locally as Sylvain Mollier, suffered gunshot wounds and was found dead close to where the bodies of a man and two women were found in a BMW in Haute Savoie.
Local paper Le Dauphine said cyclist Mr Mollier was a 40-year-old father of three, currently on paternity leave, and had gone for an leisurely cycle.
His wife, worried when he did not return from his cycle ride, alerted the authorities, without making a link to the killings.
She went to a local police station with a photograph of her husband. Officers quickly made the link and her husband was identified as the fourth victim.
He found himself in the wrong place at the wrong time, Annecy prosecutor Eric Maillaud said.
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One seven-year-old girl was critically injured and a four-year-old survived unharmed, hiding under the bodies of her parents and grandmother.
Officers were reportedly given a tip-off about the grisly scene by another cyclist.
Investigators said no weapon has been found and no arrests have been made. It was also unclear if the shootings had been carried out by one killer or a number of people.