The six-story budget hotel where 20 people died Friday in one of the capital's worst fires in recent memory was overcrowded and had only one exit, fire officials said.
Ninety people were staying at the Paris Opera hotel, built to hold 61 guests, when the fire broke out at 2:20 a.m.
The only way out was a stairwell choked with thick black smoke and flames that sent people jumping from windows before dawn.
Half the victims were children, and many were African immigrants and other people without means lodged there by authorities while they sought more permanent housing.
The prosecutor's office opened an investigation, judicial officials said, though "at this stage, we have no indication that it was anything but an accident," Interior Minister Dominique de Villepin said at the scene.
Fifty-three people were injured, 11 seriously. Police did not release the identities or nationalities of the dead. The injured came from France, Senegal, Portugal, Ivory Coast, Tunisia, Ukraine and Algeria, police said.