West Yorkshire Police confirmed they were investigating six reports of people receiving a vile letter encouraging people to take part in a 'punish a Muslim' points-scoring game.
One of the recipients of the letter, Councillor Riaz Ahmed, a Liberal Democrat councillor in Bradford said the letter he received to his business address had a Sheffield postmark.
He told The Mirror: "It was not addressed to anyone, just the address and postcode as if it was sent out randomly. It has a second class stamp on the plain white envelope and the frank mark is Sheffield. It was posted yesterday.
"It seems strange that anyone would send something like this to an address in a predominantly Muslim area. When I opened it and saw the content I was horrified."
A spokesman for West Yorkshire Police said: “Police are investigating a number of reports of potentially malicious communications sent to individuals in the Bradford area.
"Officers are currently investigating the full circumstances.
"Anyone with any concerns about a communication they may have received should contact police on 101."
Counter terrorism police are also believed to be investigating the incident.
A police spokesperson said: "Counter Terrorism Policing North East are coordinating the investigation at this time and will consider any potential links to existing inquiries.
"Anyone with any concerns about a communication they may have received should contact their local police force."
The 'Punish a Muslim' letters urge people to take part in a sick game on April 3, 2018 where points are awarded for evil acts against Muslims.
The despicable acts range from verbal abuse at the bottom of the scale to the much more disturbing acts of 'Throwing acid in the face of a Muslim', 'Beat up a Muslim', 'Butcher a Muslim' or even 'Burn or bomb a Mosque'.
The letter bears an image similar to the royal coat of arms and there is also an image of a dagger with the letters MS, suggesting it is linked to so-called Muslim Slayer who targeted mosques in London and the United States last year with similar threats.
Campaign group Tell Mama UK said: "We have been sent several reports of the 'Punish a Muslim Day' letter in London, the Midlands, and Yorkshire.
"We are working closely with the relevant police forces on this matter which is being treated with the utmost seriousness.
"It is essential that all letters and envelopes are kept and handled minimally to preserve evidence for the police to investigate.
"Incidents like this are rare but we urge vigilance and calm as we remind Muslim communities that our confidential support service is available to assist in this matter."
Iman Atta, director of Tell Mama, said: “Those behind the malicious letters are trying to create fear within the Muslim communities and the threatening letter titled ‘Punish a Muslim Day’ may be part of a wider campaign of hate against Muslims.”