The prime minister has been among people across the UK who have been targeted by a poison pen letter writer.
Police are trying to find who is responsible for more than 50 racially and sexually abusive letters.
Gordon Brown's Fife constituency office received a letter in April, with others sent to mosques, hospitals, universities and private homes.
Much of the hate mail, mostly posted from Hampshire, includes references to "repatriation" and "exit Europe".
The first letter was sent in January 2007 with the most recent sent to a private address in Havant, Hampshire, in July 2009.
They have also been received in Derbyshire, Merseyside, Lincolnshire and Bedfordshire.
These forces had initially been conducting separate investigations until Hampshire Police set up Operation Heron in January 2009 to co-ordinate the search.
One of the letters was sent to Somers Park Primary School in Portsmouth.read more
