Eric Cain says he saw the fight, but thought nothing of it because "bar fights happen all the time."

A witness has spoken to PEOPLE about the stabbing assault on Spencer Stone that took place Thursday on the streets of Sacramento – leaving the French train attack hero in hospital with serious injuries.

While doctors say Stone is expected to make a full recovery, authorities are continuing to piece together what led to the attack.

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Eric Cain, who works at a liquor store near the scene of the fight, says Stone behaved heroically as tried to help a girl in trouble moments before he was stabbed.

“A big group was walking down the street and they went into the intersection,” Cain tells PEOPLE. “One of the guys hit one of the girls and the guy who turned out to be Spencer Stone went after that guy. It happened really quickly.”

Cain adds that he saw blood on Stone’s shirt, but thought nothing of it since he walked away just fine. In hindsight, he says he should have alerted the police.

“They were all walking together from the direction of Badlands [a nightclub in the area],” he says. “Then the argument happened. The strange thing was that Spencer stuck up for the girl after she got hit and then that girl left with them [the men the police are going after].”

Stone left the scene with three stab wounds that were treated at nearby UC Davis Medical Center where doctors said that his injuries, while serious, were not life threatening.

After the stabbing, Sacramento police clarified that the incident was not a terrorist attack on the train hero.