Gunmen gesture as they return to their car after the attack outside the offices of French satirical weekly newspaper Charlie Hebdo (seen at rear) in this still image taken from amateur video shot in Paris January 7, 2015. REUTERS/Reuters TV

PARIS (Reuters) - A relative of one of the militants who attacked French satirical magazine Charlie Hebdo in Paris in January 2015 has been put under formal investigation for Islamist ties and remanded in custody, a judicial source said on Saturday.

The source said Mourad Hamyd was suspected of trying to travel to Syria to join Islamic State.

Hamyd, who is French and a brother-in-law of assailant Cherif Kouachi, was detained in Bulgaria last month after a Paris court issued a European arrest warrant against him.

Hamyd was questioned by French police following the Charlie Hebdo attacks in which Cherif Kouachi and his brother Said shot dead 12 people before being killed by police, but was cleared of any involvement.