The incident happened on Wednesday.
The woman was first called and a short time later approached at the door by two young men in their 20s, the police say. The two men tricked the 90-year-old into giving up a gold chain.
– We have spoken to both her and the family. We have received several messages in recent days about similar cases where people have contacted the elderly and say they want to buy books, says operations manager Rune Hekkelstrand in the Oslo police district to NTB.
There are no cases where people have posted advertisements to sell books online, he says.
– This does not appear to be accidental. It seems like cunning, and unfortunately nice, fraudsters, who are looking for profit. The police think this is serious, he says.
People who have been exposed to the same are asked to contact the police.
– The mode and traces these men leave behind will be used in the investigation. The more people who contact the police, the more leads we get to work with, says Hekkelstrand.