WARSAW, Poland (AP) — Polish police are searching for the fifth straight day for a small boy whose father failed to deliver him to his mother and then died after being hit by a train.

In one of the force's biggest searches ever, the police, aided by sniffer dogs and drones, have combed in vain through thousands of hectares (acres) of land and abandoned buildings around a highway between Warsaw and Grodzisk Mazowiecki, where the father's car was last seen Wednesday.

Later that day, the father of 5-year-old Dawid Zukowski was killed by a train. Media reports quoted the train's driver as saying the man threw himself under the train.

The police have been appealing to people for any information and have published images of the boy, who is both Polish and Russian, and of his Polish father.