A flight from San Francisco's airport to Chicago O'Hare saw three people hospitalized Sunday, Chicago police said.
The United Airlines flight was met by officers at the gate at around 6:02 a.m. after a "disruptive customer" was removed by law enforcement, the airline said in a statement.
The airline added that a flight crew member was taken to hospital for an evaluation.
Police said they responded "for a disturbance that had occurred on a plane."
Three occupants of the plane—including one crew member—were then transported to Resurrection Hospital for observation. The incident is currently under investigation, police said, and further details were not disclosed.
United Airlines said: "We're grateful to our crew for handling this difficult situation with professionalism and for putting the safety of our team and our customers first."
The FBI is now investigating the incident and has been contacted for comment. The airline was also contacted for an update.