French prosecutors are seeking to use a European arrest warrant to gain custody of a relative of one of the Islamist militants who attacked satirical magazine Charlie Hebdo last year, a judicial source said on Sunday.
Authorities suspect 20-year-old Mourad Hamyd - the brother-in-law of assailant Cherif Kouachi - of attempting to join the ranks of Islamic State, the source said, confirming information cited by newspaper Le Journal du Dimanche.
The newspaper said Hamyd, who was being monitored by French security services, was reported missing on July 25 by his family in France and was being detained in Bulgaria after being turned back at the Turkish border.
The Bulgarian interior ministry confirmed Hamyd was being held in the country and that a court would rule on Tuesday on his arrest.
"He had behavior typical for a foreign fighter and that's how was identified," Interior Minister Rumiana Bachvarova said on BTV television.
She said a European arrest warrant had been issued, without giving further details.
France has suffered a series of deadly attacks that have highlighted difficulties faced by the authorities in tracking potential militants.
The Charlie Hebdo attack in January 2015, in which Cherif Kouachi and his brother Said shot dead 12 people before being killed by police, was followed by a coordinated assault by gunmen in Paris in November that killed 130 people.
Hamyd had been questioned by police following the Charlie Hebdo attacks but was cleared of any involvement.
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