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Arsonists torched a police car as officers were investigating an incident.
It happened in Link Road, Canvey Island at around 3.30am on Sunday, Essex Police said.
Fire crews attended and extinguished the fire.
Detective Inspector James Freeman said: "The police car was parked while officers were carrying out inquiries and I'm appealing for anyone with information to come forward."
Anyone with information is asked to call officers at Southend CID on 101 or Crimestoppers, anonymously, on 0800 555 111.