Police in Sweden are investigating after extreme Islamist notes were posted through people's doors warning them they will be beheaded unless they convert. The notes, which are signed "ISIS", say that any non-believers will be decapitated unless they convert or pay a "jizya" (religious tax). They have been posted through letterboxes in several cities including the capital Stockholm. They carry the Islamic State banner and claim the police "will not save you from being murdered". The notes says: "In the name of Allah, the merciful, full of grace. You who are not believers will be decapitated in three days in your own house. We will bomb your rotten corpses afterwards. "You must choose between these three choices: 1. Convert to Islam. 2. Pay the jizya for protection. 3. Or else, you will be decapitated. "The police will not prevent or save you from you being murdered. (Death comes to all of you)." The MailOnline says police are now investigating the notes and have drafted in reinforcements to patrol some areas. Emil Andersson, police chief in the town of Sigutna, told Radio Uppland his officers were taking the threat seriously and that security measures would be increased. Sweden's intelligence agency is now analysing the notes, examining the symbols and language used to discover more about who could have sent them. In October, Breitbart London reported how Assyrian Christians in the city of Gothenburg were told to "convert or die" in a spate of Islamist graffiti. The local Assyrian association's president said: "Imagine yourself having fled from persecution in a country and then to find yourself staying next door to ISIS sympathizers here in Sweden. It is everyday life for many people." Yusuf Asmar, whose pizzeria was targeted by the graffiti, compared their situation to that of the Jews in Nazi Germany. "It feels extremely uncomfortable that this has come to Gothenburg, Sweden. There is no doubt that it is directed at me as a Christian and Assyrian... It feels like persecution of Jews in the 30s when Jews in Germany had Stars of David painted on its doors. Now it happens here."