"I think it has been sent out by a group with the intent of alarming people and to stir up racial hatred," a Bradford Moor councilor who received such a letter told Telegraph and Argus, a Bradford news outlet. "My main concern is that it could get into the hands of other disaffected people in Bradford who just need that push to act. Some people may look at is at a sick joke. I don't."
The letter specifies a point system for different ways to hurt Muslims on April 3.
The letter reads: "They have hurt you, they have made your loved ones suffer. They have caused you pain and heartache. What are you going to do about it? Only you can help turn this thing around, only you have the power. Do not be a sheep!," according to alaraby.co.uk.
West Yorkshire Police say they've received six reports related to the letters.
Police have started a hate crime investigation.
"Counter Terrorism Policing North East are coordinating the investigation at this time and will consider any potential links to existing enquiries," a West Yorkshire Police spokesman said.