Police in Burnley are urging householders to be on the look out for two men in their 20s offering to do building work.
It is believed the men are specifically targeting older people.
The men, one dark haired the other blond, called at a house in Woodgrove Road, Burnley, and claimed they had spotted a problem with bricks on a chimney.
They offered to do the work for £400. The men were travelling in a white Transit van and were casually dressed, but not in work clothes. Both were white and had clean clothing, and one had a scarf around his neck.
A police spokesman said: “We want people to be aware of cold callers, and for them to call us if there is anything suspicious.”