The former Kent youth police and crime commissioner has been interviewed under caution by a Special Branch officer over comments she made on Twitter.
Paris Brown, 17, who quit the £15,000-a-year post on 9 April, is being investigated over tweets which could be considered racist and homophobic.
Her lawyers have complained to Kent Chief Constable Ian Learmonth that the investigation is "disproportionate".
Kent Police said it disagreed with that interpretation of the investigation.
A spokesman said police had received more than 50 complaints from members of the public and the force was trying to establish whether any criminal offences had been committed.
In a letter to Mr Learmonth, Miss Brown's lawyers Olswang said she was visited at home by two officers, one from Kent Special Branch, on 11 April when she surrendered her mobile phone for examination.
She was interviewed under caution at a Kent police station on 14 April and her phone was returned three days later.
The letter says lawyers were "very concerned" police undertook the investigation.
It says the nature of the investigation was "wholly disproportionate and unjustified having regard to the Twitter material itself and the broader circumstances of the case".
Miss Brown was appointed to the youth post on 3 April, when she was still 16, by Kent Police and Crime Commissioner Ann Barnes to represent young people across the county.
The Mail on Sunday later reported Ms Brown had boasted about her sex life, drug-taking and drinking on her Twitter account @vilulabelle and had also posted violent, racist and anti-gay comments. The account has since been removed.
Kent Police said in a statement: "Officers must first determine the extent of the misuse of social media so that the Crown Prosecution Service can then 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."