Picture used for illustrative purpose only.

The man, born in 2001, appeared before a court in the French capital on Friday after the theft on Monday evening from a train at the capital's Gare du Nord station.

The bag belonged to an engineer from Paris City Hall, who had put it in the luggage compartment above his seat.

The crime raised concerns when a police source specified that the bag contained a computer and two USB keys with security plans for the Paris Olympics.

However, the Paris prosecutors' office later said that the USB key "contained only notes about road transport in Paris during the Olympics, and not sensitive security information".

The suspect was "already known" to police for thefts on public transport, particularly at the beginning of January, the judicial source said.

In addition to the theft as a repeat offender, the defendant was also on trial for refusing to hand over his telephone code to authorities.

The city employee is set to be investigated to see if he broke any internal security rules by travelling with the USB key, a statement from Paris city hall said.

Around 35,000 security personnel are expected to be on duty each day for the Olympics from July 26-August 11, including around two thousand municipal police officers from the capital.

As well as managing crowds and preventing street crime, police will be on high alert for possible terror plots, with France a target of Islamist extremists over the last decade.

The Paralympics are set to take place from August 28-September 8.

Agence France-Presse