Swedish police are investigating notes left in mailboxes in several cities across the country threatening those who do not practice Islam with death, according to a report by the Daily Mail.

The credibility of the notes is under investigation by Swedish authorities. The notes bear an “ISIS” signature and the terrorist group’s flag.

According to a translation by the Daily Mail, the notes read:

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).'

Jizya is a tax imposed on non-believers.

Per the Daily Mail, Sweden has accepted more than 200,000 Syrian refugees this year and “tensions have been mounting” in the country.