LITHICUM, Md. -- Southwest Airlines officials say a passenger got sick aboard a flight Thursday night before it landed at Baltimore-Washington International Airport and that set off a chain-reaction sickness aboard the plane.

Brandy King, a Southwest Airlines spokeswoman said a passenger who was ill vomited during the flight and that triggered "sympathy nausea" in three nearby passengers.

A spokeswoman for BWI said the woman who became ill was taken to a local hospital for treatment. The three other passengers declined medical treatment.

Passengers were allowed to leave the plane about 11 p.m. They were taken to a holding area and questioned in an effort to determine if anything on the plane had caused the four to be stricken, and allowed to leave the airport.

"It was a tremendous opportunity to exercise our emergency response," Assistant Maryland Transportation Secretary James Ports Jr. said at a news conference early Friday at BWI. He said he was in contact with Gov. Robert Ehrlich throughout the incident.

Earlier reports said about half a dozen of the passengers had become violently ill.