More than 700 shots have been fired at mostly car transporters on German roads since 2008, federal authorities said in Wiesbaden when announcing the reward on Tuesday. They admitted they were baffled by the case.
"We need to stop this series of crimes, so nothing worse happens," said Jörg Ziercke, chief commissioner of the Federal Criminal Police.
The reward being offered for information leading to the arrest of the shooter or shooters was raised from €27,000 to €100,000.
"We do know that in many cases the same weapon was used," Ziercke said. The federal police are also working on the assumption that the shots were fired from a truck as it passed the car transporters, Die Welt newspaper reported.
The most recent cases were at the beginning of November, when in Leipzig it was noticed that two car transporters that had come from Bavaria had been shot at.
The states with the most shooting incidents have been Bavaria, Baden-Württemberg, Hesse, North Rhine-Westphalia, and Rhineland-Palatinate.
The shooter started using a nine-millimetre-calibre gun in June - a more powerful weapon than the smaller 22-calibre gun used before.
It is not thought that the shooter has targeted people, but a 40-year-old woman was seriously injured after being shot in the throat in 2009 on the A3 motorway near Würzburg.
"It's just luck that more people have not been injured so far," Ziercke said.
A group of 90 officers from the federal and state police has been on the case since last month, Die Welt reported. One problem facing the investigators is the fact that most of the bullet holes are not noticed until the cars are unloaded from the transporters, leaving no clue as to where the shots were fired.
Shots have also been fired at car transporters in other neighbouring countries, including Belgium, France and Austria.
The police are appealing to drivers to help them search for the assailant or assailants. "We are looking for people who have noticed shots being fired on the motorways, or who know truck drivers who own weapons," Ziercke said.