What do you note about this letter's language? updated photo = more information
More than a dozen churches have been sent "threatening letters" bearing a West Midlands postmark, with one letter warning of a petrol bomb attack.
West Midlands Police is investigating and said churches and supermarkets in the region and further afield had been targeted "over a period of months".
In one handwritten letter, threats were made to "petrol bomb" services and stab congregation members "one by one".
A priest from Sheffield described the letter he had received as "horrifying".
Each letter bears a West Midlands postmark, the BBC understands.
Parish priest Father Andrew Browne, of St Mary's Church in High Green, Sheffield, received a letter on 4 January.
He said: "It looked like just an ordinary handwritten letter on the outside.
"But when I opened it and read what it said, the horror of what it could entail became clear."
Fr Browne said he was told at least 15 letters had been received by churches nationally during a Bishops' conference last week.
He added West Midlands Police took over the investigation from the South Yorkshire force after discovering all the letters originated from the region.
In a statement the force said: "West Midlands Police is investigating after several threatening letters have been sent to mainly churches and supermarkets across the region and officers are aware of a few sent further afield over a period of months.
"Enquiries remain ongoing."
The Catholic Bishops' Conference of England and Wales said it could not comment on an ongoing police investigation.