A man arrested over claims that he tormented a mother with abusive online messages is a serving police officer.
Nicola Brookes, 45, from Brighton, East Sussex, made a complaint of online harassment after receiving abuse on Facebook when she posted a comment supporting former X Factor contestant Frankie Cocozza when he left the show last year.
Ms Brookes, a single mother who suffers from Crohn's disease, said fake Facebook profiles were set up in her name using her picture to post explicit comments and that her daughter was also targeted with abuse.
Frustrated at what she considered to be a lack of action by Sussex Police to help track the perpetrators, Ms Brookes took legal action.
She took her case to the High Court in London where an order was granted compelling Facebook to reveal the IP addresses and other information of the people who had abused her.
Last week officers investigating the hacking of Ms Brookes's Facebook and email accounts said they had examined her computer to try to trace the source of the abuse and security breach.
A 32-year-old man, from Bournville, Birmingham, was arrested on suspicion of misuse of a computer by Brighton CID officers and the West Midlands Police Force.
The man, who is a police officer, has been bailed until October 19 pending further inquiries.
A West Midlands Police spokeswoman said: "West Midlands Police is assisting Sussex Police in connection with an ongoing inquiry regarding a serving West Midlands Police officer.
"The investigation is ongoing by Sussex Police, and the allegations do not relate to use of police systems.
"The officer has not been suspended. The officer is not a frontline officer."
Ms Brookes said she felt relieved that someone had been arrested but she was shocked to discover one of her alleged abusers is a police officer.
She said she was "angry and disgusted" at the people who had been "trolling" her and said she had received more than 3,000 insults through social networking.
She said: "We need something that protects people because the system that Facebook has in place does not work. I have been trolled relentlessly.
"When Tom Daley was trolled, within 24 hours someone was traced and arrested.
"When I got the call from Brighton CID to say someone had been arrested I was just so relieved and pleased. It is a significant breakthrough."
Sussex Police said they have asked Facebook to remove any abusive posts about Ms Brookes and that any such allegations of bullying, harassment or malicious communication are taken seriously.
Facebook said it shares information such as IP addresses and basic subscriber information, including names, email addresses and registration dates, when there is a legal justification and obligation.
It receives similar requests frequently but the company said all demands for information must be backed up by a court order.
A spokesman said the company respected its legal obligations and works with law enforcement to ensure that those who harass other users are brought to justice.