A passenger on an Air France flight Monday said he knew something was amiss when a woman's carry-on bag started moving around by itself.

Flight attendants opened the bag and discovered a baby inside.

Now, French investigators say they're trying to determine why the woman decided to sneak the baby on board Air France Flight 1891, without a valid ticket for the child. The flight had already left Istanbul on its way to Paris.

Police didn't name the woman. The baby was 1 or 2 years old, Metro reports.

A witness told the French website planet.fr, "The cloth bag belonged to a woman sitting at the back. At one point, she wanted to open it and somebody on the neighbouring row of seats saw that something was moving inside." A crew member said the passenger who spotted the bag was "very shocked," Metro adds.

Air France says it requested "the presence of French authorities on the flight's arrival" to deal with the matter. Flight attendants reportedly moved the woman and the baby to the front of the jet until it landed.

The Associated Press contributed to this report.