Two men have been arrested after a death threat was sent to the new manager of Oldham Athletic Football Club.
Darren Kelly was appointed boss at the League One club last week - but the 35-year-old from Londonderry caused controversy after describing former IRA chief Martin McGuinness as "a legend" in a tweet.
Police were called to the Boundary Park stadium in the town on Monday after the football club received the threatening letter.
Two men, aged 27 and 42, have been arrested on suspicion of making threats to kill and remain in custody, Greater Manchester Police said.
Detective Inspector Jason Byrne said: "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."
Some fans have threatened to boycott Oldham's matches following the decision to employ Kelly.
Oldham caused controversy in December when the club was in talks to sign convicted rapist Ched Evans, before pulling out of the deal after a number of sponsors threatened to withdraw their backing.