All hostages were released unharmed and a suspect detained after the Tuesday morning incident.
Patrick Dallennes, a top official in the Val de Marne region, said: "It wasn't a firearm he used but a fake one with gas cartridges ... without the capacity to kill."
He added: "It's made to scare... but it's not very dangerous. It was a replica and at first it was hard to tell that it was fake."
The alleged hostage-taker was incoherent in his talks with police and his motivations remain unclear.