Five letters were received yesterday at post offices in Burnley, Padiham an Pendle, from a racist group calling themselves the British Liberation Militia.
The hateful letters feature swastikas and include gruesome illustrations of a lynching and a murdered man with an axe in his back.
The letters also threaten extreme violence against Lancashire's BAME communities, including murder and burning down people's homes and businesses.
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Specialist officers have reviewed the letters and have spoken with members of staff who the letters appear to have targeted.
But police say at this time there is not thought to be any direct threat to any individuals or the wider community.
The letters are thought to be linked to previous letters which were received by post offices across Lancashire in 2019.
A police spokesman said: "We are investigating a number of threatening letters received by post offices in the East Lancashire area.
"We understand that this may cause concern and we want to assure people that we are working closely with the post offices involved to ensure the safety of staff and customers.
"We are looking at the possibility that the five letters reported to us yesterday are linked to a number of others that were received by post offices across the county in December last year. We are still investigating those incidents.
"We want to reassure people that we know how concerning these letters are but we are determined to find who is responsible for sending them."
If you have any information about any of the letters, you can contact Lancashire Police via email at [email protected] or call 101 quoting incident reference LC-20200624-0489.
Alternatively, you can contact independent charity Crimestoppers anonymously on 0800 555111 or online at crimestoppers-uk.org.