Thousands of litres of diesel have been stolen from two filling stations in Hertfordshire in raids over a week-long period.
In the most recent incident 20,000 litres was stolen from a BP station owned by MRH in London Road, Buntingford.
The thieves stole 11,000 litres under the cover of darkness between 11pm on Saturday May 17 and 4am the following morning, and returned the next night to steal a further 9,000 litres.
And at the garage in Cambridge Road in the nearby village of Puckeridge diesel worth £8,000 was stolen over the previous weekend. On the night of Friday May 9 diesel worth £7,000 was stolen with another £1,000 taken the following night.
Detective Inspector Phil Moss of Hertfordshire police said that after the second offence at Puckeridge staff had been on the lookout and they disturbed the raiders when they returned for a third time on Thursday May 15, and they escaped in a stolen van.
Police later recovered a van containing three 1,000 litre containers and a garden pond pump.
At both garages the access to the tanks was out of sight of staff and the criminals had been at work even while the Buntingford garage, which operates 24 hours a day, was open, said DI Moss.
He said that at Buntingford the criminals had accessed an adjacent field and laid pipes across it so they could work out of sight of the garage.
The theft was only detected on the Monday morning when the tank ran dry, and by looking back at readings from the tank the large extractions over the previous two nights were discovered.
DI Moss said police patrols in rural areas had been stepped up following the thefts and they were following some positive lines of inquiry.