Two security checkpoints were closed and several flights delayed at Baltimore-Washington International Thurgood Marshall Airport on Monday morning after what was believed to be pepper spray was released at a checkpoint on concourse B, airport officials said.

There was no indication that the chemical was released intentionally, and airport officials said they remained uncertain as to its origins.

Fire and rescue units were called to the concourse about 8:45 a.m. in response to reports of an irritating odor, according to airport spokesman Jonathan O. Dean. The A and B concourses were evacuated, and Southwest Airlines and Air Tran Airlines delayed departure of several flights until the concourses reopened.

Dean said the A concourse checkpoint opened at 9:25 a.m. and the B concourse checkpoint opened about 20 minutes later.