Statement, which has not been published, is of "violent and sometimes racist" and was found on a Facebook group entitled 'cop-info.com Forum', according to a source close to the investigation.
The group, which has now been closed, described as "non-official" forum for national and local French police.
Offensive comments believed to be related to the riots in the Paris suburb of Trappes, TFI reported on Tuesday.
Riots erupted over the weekend after a man was arrested for allegedly assaulting a police officer who had stopped her for wearing a veil - a practice that has been banned in France since 2011.
The police should be exemplary
Recent allegations of racist comments that appear to come from a member of the French police more likely to cause tension between youth and police.
Under the direction of the Ministry of Interior, the Inspector General of the French National Police (IGPN) will now try to identify the author's comments.
"The investigation will determine whether the police made the statement, in this case unacceptable and disciplinary action will be taken. But we have to be very careful and find out in what context the statement was made," a spokesman of the Ministry of the Interior Pierre-Henry Brandet as quoted by TF1.
"The police should be exemplary, whatever the circumstances. Characteristically example is very important and very necessary if we want to rebuild or revive the bond of trust that should unite security forces and citizens