A search for a missing five-year-old boy in Poland has stretched into a sixth day in what authorities have described as the largest to ever take place.

The hunt for the boy, Dawid Zukowski, began after his father failed to deliver him to his mother and then died after being hit by a train. The boy's father was last seen driving on a highway on July 10 between the town of Grodzisk Mazowiecki and the boy's mother's home in Warsaw, approximately 20 miles away.

Later that day, the boy's father was reported dead. Media reports quoted the train's driver as saying the man threw himself under a train.

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Before killing himself, David's father sent a text message to the boy's mother saying "you will never see your son again," Poland's Radio Zet reported.

Authorities have deployed sniffer dogs and drones in addition to combing through more than 7,400 acres of land and abandoned buildings around the highway where the father's car was last seen.

"This is the biggest search in the history of the Polish police," Warsaw's police spokesperson Sylwester Marczak said Monday.

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Polish police are also working with soldiers from the territorial defense force and German police as they try to find the boy, Sky News reported.

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On Tuesday, a Warsaw police spokesman stressed that authorities are still looking for the boy and dispelled media reports that the boy's blood was found on the father's clothing, saying that further testing is needed, Polsat News reported.

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.

The Associated Press contributed to this report.