The fatal shooting in the southern town of Mende appears to have been an accident, though ballistic tests and an autopsy must still be carried out, the prosecutor said.
These tests, according to Finielz, would among other things show whether the bullet had ricocheted off a wall before killing the child, rather than hitting him directly.
A 13-year-old boy is suspected of firing the shot, while a 15-year-old was being questioned on suspicion that he tried to hide the weapon after the deadly incident.
The two teenagers were known to police and live in the same building as the shooting victim's family.
Finielz said the three boys often played together, and that the two who were detained were "devastated" by the shooting death.
The nine-year-old victim, whom neighbours said was called David, was shot in the chest by a 0.22 bullet, the calibre used by many air pistols.
Police sealed off the area and an autopsy was to be carried out on Thursday.