Police in Sussex say they do not have enough evidence to prosecute anyone after a pig's head was thrown into the car park of a mosque in Crawley.
Three men were arrested on suspicion of a racially or religiously-aggravated public order offence after the incident at Langley Green mosque on 17 April.
People at the mosque said the head was thrown from a white car.
Police said there had been an intensive investigation but there was too little evidence for a prosecution.
A 21-year-old man from Horley was arrested nearby on the same evening and two other men from Crawley, aged 20 and 25, were held the following day.
Police said a car was seized for forensic examination during the investigation.