Youssef Ettaoujar, 28, a Frenchman of Moroccan origin, was convicted in 2014 along with two others.
He finished serving his sentence in October and was under house arrest as part of France's state of emergency, in place since the November 13 Paris attacks that killed 130, the judicial official said, on condition of anonymity due to the ongoing investigation.
Authorities have until Sunday to free or charge any or all of the four.
The trial was notable because it was the first in France of potential jihadi fighters in Syria when French youth were heading at an alarming rate to the battlefields of the Islamic State group.
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Ettaoujar was considered the leader among the three arrested in 2012 at a French airport as they prepared to board a plane for Turkey, the leading route to neighboring Syria. Interior Minister Bernard Cazeneuve, referring to Ettaoujar without naming him, said Wednesday night that "he might commit violent actions in France."
At his trial, Ettaoujar denied he planned to fight in Syria, claiming his mission was humanitarian. Ettaoujar was given a five-year prison term with one year suspended for "criminal association with intent to commit terrorist acts.