A serial arsonist is believed to be at large after several fires were started in Dereham.
Police have launched an investigation after the latest incident in the car park of Nelson Place.
The fire was started just before 6am on Monday.
A spokeswoman for Norfolk's police said: "We are working with the landlords and the borough council on a longer-term solution to prevent further arsons from happening.
"We have increased our patrols of the area, and we are aware that these crimes are occurring both day and night, but we do not have the resources to be there all the time.
"We are also investigating behind the scenes to identify those responsible."
Justin Dack, who volunteers for the Dereham Community Litter Pickers, said this was the third to have been started near Nelson Place during June and July.
He said there were also remnants of fires found on June 15 and June 26.
"The fires are worrying for many in Dereham and also the shop owners in the complex.
"It's only a matter of time before a building will go up in flames.
"I have cleaned up the remnants of several fires up there lately, including burnt school books, which I have managed to retrieve the students' name on the books and have reported it to the school.
"These are three separate fires in the same place, hence why people are getting upset, but nothing appears to be happening to stop it."
Anyone with information should contact the police on 101, quoting crime reference 36/53329/24, or via email to SNTDereham@norfolk.police.uk.
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