The cause of the accident is not known. The tram driver has 14 years of experience and was sober. Probably, the boy's leg was caught in the door when he and his grandmother were getting out of the tram.

Warsaw Trams on Twitter issued a statement regarding the accident: 

 "We are shocked by the tragic accident that took place today in Praga-Północ. As a result of it, a several-year-old child lost his life. We make every effort to help the services to explain the causes of this unimaginable tragedy as soon as possible."