The prayer hall in Lagny-sur-Marne, around 30 kilometres (18 miles) east of the capital, was shut down last Wednesday following a large-scale police operation.
Associated with the traditionalist Salafist branch of Islam, it is the third mosque in France to be closed after the coordinated jihadist attacks on Paris on November 13.
The prefect -- the highest representative of the state -- in the Seine-and-Marne department said Sunday "7.62mm ammunition for a Kalashnikov rifle and propaganda videos" for the Islamic State group had been found in raids linked to the closure of the prayer hall.
The locations of the raids were not given.
A revolver and jihadist documents were also found during searches at the homes of the mosque leaders, Interior Minister Bernard Cazeneuve said previously.
A total of 22 travel bans and nine house arrests for "radicalised individuals" have been issued as a result of the operation.
Police also found recordings of religious chants "glorifying the martyrs of jihad linked to the terrorist organisation Jabhat al-Nusra", the Syrian branch of Al-Qaeda, the prefecture added.
The recordings were found among a wealth of teaching material for youngsters in an undeclared madrassa, or religious school.
"No request was made to open a private school," the prefecture said in its statement.
Mohammed Ramdane, president of the local Muslim association in Lagny, had criticised the closure of the prayer hall on Wednesday, saying: "Nothing has been found. Nothing is hidden, we don't hide anything."