Two of the tourists were injured in the face in the attack in the city's main Saint Charles train station and one of them has a possible eye injury, a spokeswoman for the Marseille prosecutor's office told The Associated Press in a phone call.
She said all four of the tourists, who are in their 20s, have been hospitalized, two of them for shock. She said a 41-year-old female suspect has been arrested.
The spokeswoman did not release any further details about the victims or the suspect. She spoke on condition of anonymity, per the French judicial system.
There was no immediate information on where the U.S. tourists were from.
Marseille is a port city in southern France that is closer to Barcelona than Paris.
In previous incidents in Marseille, a driver deliberately rammed into two bus stops last month, killing a woman, but officials said it wasn't terror-related.
In April, French police say they thwarted an imminent "terror attack" and arrested two suspected radicals in Marseille just days before the first round of France's presidential election. Paris prosecutor Francois Molins told reporters the two suspects "were getting ready to carry out an imminent, violent action" on French territory.
In January 2016, a 15-year-old Turkish Kurd was arrested after attacking a Jewish teacher on a Marseille street. He told police he acted in the name of the Islamic State group.
Stay with ABC13 Eyewitness News for more on this developing story.
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