PARIS — An American who had been scheduled to appear as a star witness at the trial in Paris of an Islamic State operative was hospitalized Thursday in the French capital, casting doubt on whether he would still be able to testify about his role in the dramatic capture of the gunman that averted a possible mass slaughter aboard a high-speed train.

Spencer Stone's lawyer, Thibault de Montbrial, wouldn't give details about what is ailing the former U.S. airman, citing medical privacy. He said Stone would not testify as planned on Thursday afternoon.

"I know that he is hospitalized. I don't know why. I don't know how he is," de Montbrial said.

"The only thing I'm certain of is that he is not in a state to testify today. We are going to regroup this evening to gauge whether he can be heard tomorrow morning or afternoon."

Stone, then a 23-year-old, was among passengers who helped subdue gunman Ayoub El Khazzani aboard the Amsterdam to Paris train in 2015.

Their heroics during the drama on Aug. 21, 2015, inspired Clint Eastwood to direct a Hollywood re-enactment: "The 15:17 to Paris."

El Khazzani, a 31-year-old Moroccan, is on trial with three suspected accomplices. He risks life in prison if convicted of attempted terrorist murder. 

El Khazzani boarded the train in Brussels armed with a Kalashnikov, nine clips with 30 rounds each, an automatic pistol and a cutter, according to investigators. Once aboard the train, El Khazzani lingered in a restroom between cars and then emerged bare-chested with his weapons. 

One of the witnesses scheduled to testify on Thursday afternoon is Mark Moogalian, a French-American who wrestled the Kalashnikov off El Khazzani — before being shot himself by a pistol. 

Stone has said he was coming out of a deep sleep when the gunman appeared. He and Alek Skarlatos, then a 22-year-old U.S. National Guardsman recently back from Afghanistan, snapped into action, tackling the gunman.

Skarlatos was scheduled to testify Friday.

Stone, whose hand was injured by the cutter, is also credited with saving Moogalian, whose neck was squirting blood.