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 women, who are in their 20s, have been hospitalized, two of them for shock.
The spokeswoman said the 41-year-old female suspect did not make any extremist threats or declarations during the attack. She said there were no obvious indications that the woman's actions were terror-related, but added that officials could not be 100 percent sure about ruling out terror links at such an early stage of the investigation.
She did not release any further details about the suspects or the victims, including where in the United States the tourists were from.
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The Marseille fire department was alerted just after 11 am and dispatched four vehicles and 14 firefighters to the train station, a department spokeswoman said.
Two of the Americans were "slightly injured" with acid but did not require emergency medical treatment from medics at the scene, the spokeswoman said. She requested anonymity in keeping with fire department protocol.
A spokesman for the United States embassy in Paris said the US consulate in Marseille was in contact with French authorities about the attack investigation and the condition of the American women.
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.
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