Dr Tim Grant said women tend to be less direct in their threats
The letters are filled with anger and abuse.
Recipients, which have ranged from mosques to the prime minister's office, have been bombarded with race hate and sexual insults.
But perhaps the most surprising thing about this case, which involves 57 letters and is being investigated by police, is the culprit could be a woman.
Linguistic experts have studied the letters and say a range of factors point to female involvement.
Dr Tim Grant, deputy director of the Centre for Forensic Linguistics at Aston University, Birmingham, has been involved in the analysis.
He said: "A lot of people assume politically motivated hate mail is written by men.
"Men tend to suggest a more explicit threat and a demand for action but, while the nature of the letters were very nasty and would clearly have been received as threats, they were not explicit about what that threat might be."
For many months police around the country had treated the incidents as separate but in January 2009 Hampshire Constabulary took over the investigation after it was discovered all 57 letters were sent from the Portsmouth and Southampton area.
The Centre for Forensic Linguistics regularly helps with police investigations, including counter terrorism cases.
Dr Grant worked on the case of Dhiren Barot, who was jailed in 2006, proving he had written documents planning terrorist attacks in the UK and US.
Some of the letters have featured similar cartoons
He has built a profile in this latest inquiry using well-documented differences between the way men and women use language.
Dr Grant has studied the letters, which are also emblazoned with cartoons and drawings.
He said: "One of the things that were striking about the letters was the heavy use of expressive adjectives, which is more typical of women than men.
"You could say women use more adjectives because they can be more socially evaluative but we don't look at why rather than how the two different groups behave.
"We just know that's the case because we read a lot of letters and make statistical correlations. The words (in the letters) used were things like 'squalor', 'dirty' and some sexual adjectives which were suggestive of women's writing.
"Another thing we know is that women tend to use fewer first person pronouns, such as 'I'."
The investigation featured on BBC1's Crimewatch programme when Dr Grant revealed his profile of the suspect.
He said "With any profiling there are always uncertainties so we would not want to stop people phoning in if they thought the letter writer was a man.
"These are statistical correlations but it all adds up consistently, which is why we were happy to say it on Crimewatch.
"Forensic DNA evidence on eight of the letters was all the same, which was female DNA. I am not involved in that aspect but you see the pattern was building up.
"There were some unusual phrases in the letters like 'ancestral English beds' and 'ancestral English hospitals'," explained Dr Grant.
"They also referred to 'spiritual woodlands and meadows'.
"One of the things I do when I see a phrase is to look it up on Google or linguistic databases, which could help identify certain groups. Finding a phrase like that can help you track down a single social group.
"It's unlikely that those were the only letters that the person has written. They may also be writing letters of complaint to supermarkets or writing to the council. With those unusual phrases, there may be someone who can help us identify them."
Meanwhile, police are sifting though information from 86 calls which they received after the case was featured on BBC One's Crimewatch on Tuesday.