Airman 1st Class Spencer Stone, celebrated as a hero for helping to stop a terror attack on a French train over the summer, was stabbed and seriously wounded outside a bar in his hometown early Thursday.
Stone, 23, was knifed repeatedly in the upper body but was expected to survive, authorities said. He was taken to UC Davis Medical Center.
"This incident is not related to terrorism in any way," Deputy Police Chief Ken Bernard said. "We know it's not related to what occurred in France months ago."
A grainy surveillance video from a camera outside a store showed a man who appeared to be Stone fighting with several people at an intersection. The group spilled into the street as people took swings at each other, and one person got knocked down.
Police said two assailants fled in a car. No immediate arrests were made.
Bernard said Stone was out with four friends when they got into a fight with another group of people. The deputy chief would not say what sparked the argument. He said there was no evidence the assailants knew who Stone was.
In a statement, the hospital said Stone's family "appreciates the outpouring of love and support" and requests privacy.
In August, Stone and two of his childhood friends from Sacramento, National Guardsman Alek Skarlatos and college student Anthony Sadler, were vacationing in Europe when they sprang into action aboard a Paris-bound passenger train and tackled Ayoub El-Khazzani, a man with ties to radical Islam. He had boarded the train with a Kalashnikov rifle, a pistol and a box cutter.
Stone, who is assigned to Travis Air Force Base in California, suffered a severely cut thumb and a knife wound to his neck during the struggle with the gunman.
President Barack Obama met with the three Americans last month, praising them for their quick thinking and courage and calling them "the very best of America." They were also awarded France's highest honor by President Francois Hollande. The three appeared on late-night talk shows and received a parade in their hometown.
Stone is the second of the three Americans to be shaken by violence at home since their return.
Last week, Skarlatos left rehearsals for TV's "Dancing With the Stars" to rush back to his hometown of Roseburg, Oregon, after a gunman killed nine people at the community college that Skarlatos attends.
"It's honestly the strangest emotion I ever felt," Skarlatos said in a taped segment that aired on the show Monday. "Even the train made more sense than this does. ... There's nothing you can do."
The stabbing happened in a busy area of central Sacramento ringed with bars and restaurants that is a popular nightlife destination for young adults and is generally considered safe.
Skarlatos tweeted Thursday: "Everybody send prayers out to the Stone family today."
Associated Press writer Juliet Williams in Sacramento contributed to this report. Watson contributed from San Diego.