A Spirit Airlines flight preparing for takeoff at BWI Thurgood Marshall Airport returned to the gate just as it was taxiing to take off Tuesday.

The plane was later cleared for takeoff, and left for Chicago just after 9 a.m.

Maryland Transportation Authority Police 1st Sgt. Jonathan Green said the Spirit flight No. 969, which was headed for Chicago, returned to the gate around 6:30 a.m. due to concerns from the flight crew. The flight was scheduled to depart BWI at 6 a.m. 

Green said four people were removed from the plane for questioning.

There is no word on what concerns led to the plane returning to the gate or why the passengers were removed.

Spirit Airlines spokesman Stephen Schuler released the following statement on the incident:

"Spirit Airlines flight 969, scheduled for a 6 a.m. departure from Baltimore-Washington International Airport (BWI) to Chicago O’Hare international Airport (ORD), experienced a delay this morning due to law enforcement activity. While the aircraft was taxiing to the runway, a passenger alerted a flight attendant of a passenger engaged in suspicious activity on board.

"The flight attendant notified the captain, and out of an abundance of caution, the plane returned to the gate. Law enforcement officers removed four passengers from the plane for questioning. All passengers were deplaned for a brief time. The aircraft and luggage has been cleared by the TSA and the plane is scheduled to take off shortly. All questions regarding the investigation should be directed to the TSA and Maryland Transportation Authority Police." 

Schuler said the remaining passengers were let off the plane for a brief time, while the aircraft and luggage was searched.