Lawyers for Paris Brown, the youth police and crime commissioner who resigned after criticism of comments she made on Twitter, have criticised the police for their "wholly disproportionate" response.
The 17-year-old had her mobile phone seized and was interviewed under caution by Special Branch investigating the material, they said.
Her questioning by officers came after Kent Police received more than 50 complaints from members of the public amid claims that the tweets were homophobic, racist and violent.
Paris Brown resigned from her £15,000 a year role
She was quizzed for one hour and seven minutes about material posted on her Twitter account which had already been published in the media. Her phone was returned three days later.
On Saturday it emerged that Paris's lawyers, Olswang, have written to Kent Police Chief Constable Ian Learmonth about the scope and nature of the investigation, including the decision to seize her phone and for Special Branch to quiz her.
In the letter, her lawyers said: "That appears to be wholly disproportionate and unjustified having regard to the Twitter material itself and the broader circumstances of the case."
They said Paris, from Sheerness, was visited by a Special Branch officer at her home on April 11, along with a second officer, and asked to surrender her mobile for examination.
Then she was requested to attend an interview under caution on April 14.
In their letter, her lawyers said: "As we are sure that you will readily understand, being subject to a police investigation is highly distressing for any person, but especially so for a teenager, particularly one who has been recently subject to such adverse media coverage.
"In such circumstances, we believe that the police must weigh carefully the extent to which an investigation relating solely to social media activity is merited."
Olswang also referred to the interim guidelines from the Director of Public Prosecutions (DPP), Keir Starmer, regarding prosecutions relating to social media.
Paris's lawyers said it was "immediately apparent" from reviewing the Twitter material that it would be "inconceivable" that they would form sufficient grounds for a prosecution.
"There is nothing in the Twitter material which could be deemed, even on its face, to be grossly offensive or otherwise unlawful," the letter added.
"There was therefore no proper basis on which to require our 17-year-old client to surrender her mobile telephone, to caution her or to subject her to a detailed police interview whilst under caution."
Kent police and crime commissioner Ann Barnes, who appointed Miss Brown to the position, said earlier this month that the police investigation into the social media posts was "the straw that broke the camel's back".
But she praised the teenager as a "remarkable young woman" and said her decision not to take up the role, which was due to begin this summer, was "very sad".
Mrs Barnes, who was to pay £5,000 of Paris's £15,000 salary from her own pay, admitted that the teenager's Twitter feed had not been vetted as part of the recruitment process.
Ann Barnes said there were lessons to be learned
Another youth commissioner will be appointed later this year, but Mrs Barnes said there were "lessons to be learned" before someone replaces Paris.
A police spokesman said today: "Kent Police received more than 50 complaints from members of the public and are currently investigating to establish whether any criminal offences have been committed in relation to this incident.
"Kent Police disagrees with the interpretation of the investigation given.
"Officers must first determine the extent of the misuse of social media so that the Crown Prosecution Service can then, taking into account the DPP's interim guidance, determine whether or not any potential evidence meets the set criteria for a prosecution.
"We will investigate all complaints thoroughly but cannot comment further on the progress of a live investigation."