Oldenburg, Germany  - German police arrested a 30-year-old drug user Wednesday, accusing him of hurling a log off a bridge over a highway and killing a mother in the passenger seat of a moving car.

The March 23 killing led to a spate of copycat attacks all over the German highway system. Police say little can be done to prevent such anonymous attacks on speeding traffic at night time.

Nikolai H had admitted deliberately throwing the 6-kilogram chunk of wood, which smashed through the windscreen of a family's car on autobahn number 29 near the northern city of Oldenburg, police said.

The woman, 33, was killed nearly instantly. Her husband, 36, daughter, 9, and son, 7, were in the car with her, bound homewards after an Easter seaside holiday on the Baltic coast.

The accused had come forward on April 5, claiming to police and television news channels that he had discovered the log lying on the road before the attack and shifted it to the roadside so it would not be an obstruction.

Police did not believe the Kazakhstan-born German man and found similar cut logs with similar dirt on them at the accused's home.

Under questioning, the unemployed man, who has served prison time for theft, broke down and admitted he had thrown the log. Senior detective Reiner Gerke said the accused described his motive as "frustration."

In many places there is no speed limit on German superhighways and the traffic moves at well above 130 kilometres per hour. (dpa)