The body of an elderly woman was found in car submerged in a dyke at Billingborough Fen on Wednesday afternoon.
Police were called to Main Road at about 4.30pm after receiving reports of a vehicle submerged in a dyke.
When they arrived they found a silver Vauxhall Astra in the water. The driver, an 86-year-old local woman, had died.
It is not clear at this stage how long the vehicle had been in the dyke but officers are appealing for anyone who thinks they may have seen the car in the area over the last couple of days to get in touch with them as soon as possible.
Anyone with any information should call PC Robin Dunn on 101 number, quoting incident number 288 of May 15.