The 25-year-old was given an eight-month jail term last month after being found guilty of conspiracy to defraud.
Ranger now faces a probe from the prison and the Ministry of Defence because it is illegal for prisoners in British jails to post from social media accounts while behind bars or to have mobile phones.
It is unclear whether it was Ranger who uploaded the post, but if he is found to be guilty he could be handed a two-year prison sentence.
Courtesy of Daily Mirror
The Instagram post read: “Sad to hear the news about my brother Cheick Tiote.
“He would go through a brick wall and give you a two footed tackle in training a day before a cup final!
“What a guy though, top man and a professional. My condolences go out to his family.”
A spokesman for the Ministry of Defence told the Southend Echo: “This behaviour is unacceptable and we are working to get the content removed from social media. The prison is also investigating.
“It is a criminal offence to bring a mobile phone into prison, or transmit sounds or images from within a prison using a mobile phone. These offences carry a maximum penalty of two years in prison.”
Ranger, who played 25 times last season, scoring eight goals, is still a Southend player after signing a contract last summer until 2020.