The airport tweeted that the six people were taken to local hospitals for life-threatening injuries. The Jet Bridge, an extendible aisle that extends from the terminal to the aircraft, was located at Southwest Airlines Gate 810, according to the airline.
Southwest Airlines said the flight arriving from BWI from Punta Cana, Dominican Republic, requesting paramedics to investigate the "boarding situation of a passenger". While paramedics helped the passenger outside the aircraft, the airline said the jet bridge had "experienced a mistake".
The remaining passengers boarded stairs from the aircraft.
"We are working swiftly to support all passengers and crews," Southwest said in a statement to CNN.
The BWI Fire Department and Rescue Department responded together with mutual help, the tweet added.
The jet bridge Another failure of BWI Airport is "investigated", according to another tweet from the airport.
The bridge remains closed until the investigation is completed, the tweet said. The plane had been pulled off the gate and it had no further impact on operations.