During a routine check at the Kiefersfelden border crossing, police found “explosive-like” materials in a Polish-registered car. The discovery triggered a major security operation.
The police detained the Polish driver and his three African passengers, from Ivory Coast and Guinea.
The A93 motorway was later reopened.
Three pipes were found in the car which police suspect were intended for pipe bombs. They also found a large amount of gunpowder in the car, as well as connecting wires and several mobile phones.
The Polish driver’s intentions remain a mystery. He is suspected of trafficking the Africans into Germany illegally.
German police have stepped up checks on vehicles entering from Austria since last year’s unexpected influx of 1.1 million migrants and refugees.
The security forces raised the alert level after a number of gun, bomb and machete attacks in Germany during the summer, including a mass shooting in Munich.