The officer-in-charge of Basanti police station Koushik Kundu was injured when a stone thrown by the mob hit him on the head, police said.
He was rushed to the hospital bleeding. There were, however, no arrests.
The nine-year-old had gone missing after she left her home at Kumirmari to buy some provisions on Saturday evening. There was a thunderstorm and she did not return.
Police early today arrested three of the accused - the youth, his father and his sister-in-law, while his mother, the fourth accused, is absconding.
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Trouble erupted when the arrested were being taken to Alipore court in Kolkata. The police station was gheraoed by the mob who demanded that the three be handed over to them.
They threw stones and injured Kundu. Later, however, the police persuaded them to withdraw.