Spencer Stone, hailed as a hero after a French train terror attack, has been stabbed in Sacramento, California.
He was attacked while defending a friend, and is currently in a stable condition at a local hospital, an Air Force official said.
Police said the stabbing took place on a Sacramento street corner in the early hours after an argument escalated.
Sacramento Police Officer Traci Trapani said no arrests had been made and they did not yet have a description.
She said that police are currently attempting to interview witnesses and review surveillance footage.
They hope to release more information later today.
Mr Stone, 25, is one of three Americans that helped to thwart a terror attack on a French train in August.
They were hailed as heroes for tackling a man who was carrying a Kalashnikov rifle, an automatic pistol with ammunition clips and a box cutter knife.
The men, assisted by other passengers, threw him to the floor of the carriage, removed his guns and restrained him.
Mr Stone was slashed by the attacker and later required surgery to reattach part of his hand.
Who are the six France train heroes?
He and the two others, Alek Skarlatos and Anthony Sadler, were hailed by French President Francois Hollande and greeted as heroes upon their return to the US.
They received awards from Secretary of Defense Ashton Carter and met President Barack Obama at the White House.
Investigators said on Thursday they did not expect Mr Stone to survive the attack in Sacramento and initially treated the crime as a homicide.
Mr Skarlatos tweeted "Everybody send prayers out to the Stone family today".