Police have arrested two men on suspicion of making death threats, in a letter addressed to Oldham Athletic, believed to be directed at manager Darren Kelly.
The Northern Irishman, 35, was appointed as the Latics boss last week and has been targeted after he allegedly called IRA commander Martin McGuinness a 'legend'.
He has since then refuted claims that he is a supporter of the IRA.
Detective Inspector Jason Byrne confirmed the news of the arrests: "The letter sent to the club made some extremely serious threats to a number of individuals and we are treating this matter with the utmost seriousness.
"We are also aware of equally vitriolic comments that have been made on social media and this too forms part of our investigation.
"We have made two arrests and anticipate more. Another man knows we are keen to speak to him and I would stress that it is in his best interests to come forward.
"I would also ask anyone who has information about the letter or those behind it to contact us."