Eleven members of the same family from the French city of Nice are believed to have left for Syria, with some suspected of planning to fight alongside jihadists, a judicial source said on Wednesday.
The 11 are said to include a man, his two sisters and mother, along with their respective spouses and children, including a baby.
The family is believed to have left at the end of September.
The wife of the man was identified by the Nice-Matin newspaper as 27-year-old Andrea, who left with her two sons aged four and six.
"All is well. I am in a superb region," she told her parents, according to their account in the paper.
She also confided to a friend: "I was not going to go, but before Allah I could not stand back."
Her father told the paper she had converted to Islam.
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"I saw how religion played a bigger and bigger part in her life," he said. "Perhaps I should have reacted."
The Muslim father of Andrea's husband, Reda, confirmed his entire family had disappeared, including his wife, twin daughters and four grandchildren.
"We were married since 1983," he told Nice-Matin.
"I knew religion held an important place for them, but only up to a point. My whole family has disappeared, and I'm scared."
The anti-terrorist section of the Paris public prosecutor's office has opened an investigation into the disappearance of the family, acting on a "stack of evidence" pointing to their departure to Syria, the source said.
France has Europe's biggest Muslim population and is thought to have provided the largest contingent of Western jihadists to the conflict in Syria.
More than 70 similar investigations have been launched since the start of the year.
Prime Minister Manuel Valls told MPs last month that around 1,000 French citizens are involved in jihadist networks, with an estimated 580 having travelled to fight in Syria and Iraq.
A new counter-terrorism law was passed last month banning the departure of those suspected of leaving to join jihadist movements.