Police were criticised for excluding antisemitism as a motive soon after the death of Mr Hadjadj on Tuesday 17 May, saying the killing followed an argument between the two men.
But now the Lyon prosecutor has opened antisemitism as a line of enquiry.
The decision comes after campaigning Jewish lawyers had uncovered a number of allegedly antisemitic tweets posted by the 51-year-old neighbour in recent years and sent them to police.
The posts suggest he had become drawn into conspiracy theories involving Israel and Jews. A number of tweets used the term “Sayanim” to accuse Jewish and non-Jewish figures of being undeclared agents of Mossad.
Prosecutor Nicolas Jacquet said: “Social media posts shared with the prosecution have brought it to widen the scope of the magistrates’ investigation to include the aggravating circumstances of the act being committed due to the victim’s ethnicity, nations, race or religion.”
The decision allows investigators to examine the suspect’s phones and computers to check his background and the possibility that he was radicalised. They can also now consider whether the killing was premeditated.
The victim had lived on the second storey and is said to have been a frequent visitor to his neighbour.
According to newspaper Le Progres, the 51-year-old told investigators he had heard voices telling him to kill Mr Hadjadj.
The case has disturbing echoes of the murder of 64-year-old Sarah Halimi, who was thrown off her balcony by her neighbour in Paris in April 2017.
After what was widely considered a botched investigation, her killer, Kobili Traore, was found not guilty by reason of temporary insanity because he was under the effects of cannabis.
Antisemitism was ruled out as a motive at a very early stage, even though Traore had cried out: “I killed the devil” as he murdered the only Jewish resident in the block. Police were criticised for not even examining Traore’s mobile phone.
After René Hadjadj's death, Francis Kalifat, the head of Crif, the French-Jewish umbrella group had called on prosecutors to examine if the killing was a hate crime, tweeting: “To avoid another legal fiasco like the one of the Sarah Halimi case, the investigation must this time be as complete as possible and the antisemitic motive must be examined from the start, and eventually be excluded at a later stage.”
