US Air Force airman Spencer Stone, one of the three Americans who thwarted an August train attack in France, was hospitalised after being stabbed several times during a fight outside a Sacramento bar.
The fight was not related to Mr Stone's role in taking down the gunman on a train headed to Paris and there was no connection to terrorism, Sacramento Deputy Police Chief Ken Bernard told a news conference.
Mr Stone's assailant, who has not been caught, is not believed to have known his identity, Mr Bernard said.
Mr Bernard would not say what caused the fight that led to "significant injuries" to Stone, who was taken to UC Davis Medical Center in Sacramento.
"At approximately 1am, Airman First Class Stone was involved in a stabbing incident in Sacramento," Air Force spokeswoman Rose Richeson said in a statement.
Police said the altercation began outside a bar in the California capital's midtown district.
"This incident is a very unfortunate altercation between two groups of folks who were enjoying the nightlife in midtown Sacramento," Mr Bernard said.
Mr Stone and two friends, Anthony Sadler and Alek Skarlatos, who is part of the Oregon Army National Guard, subdued a gunman on a high-speed train bound for Paris in August, winning accolades from the presidents of France and the United States.
The three men, who grew up in the Sacramento area, were touring Europe, partly to celebrate Mr Skarlatos' return from a tour of duty in Afghanistan, when they took down the man armed with a box-cutter, a pistol and an AK-47 assault rifle.
They were feted back home in Sacramento with a parade.
After the fight on Thursday morning, police initially did not believe Mr Stone would survive, but doctors were able to control his bleeding, the Sacramento Bee reported. The medical centre said Mr Stone was in serious condition.
Stone, who works in a medical unit, was recently transferred to Travis Air Force Base near Sacramento and has been slated for a promotion to staff sergeant, Air Force spokeswoman Captain Brooke Brzozowske said.
