'Unknown substance' reported on Southwest aircraft turns out to be tissue box fibers

An “unknown substance” that forced a plane to evacuate upon landing at Baltimore/Washington International Thurgood Marshall Airport (BWI) turned out to be nothing more than fibers from a tissue box.

The incoming Southwest Airlines flight, which originated in San Antonio, Texas, was deplaned safely on Monday night after the substance was found during the flight. Local authorities were also contacted to meet the plane upon landing “out of an abundance of caution,” the airline confirmed to Fox News.

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A hazmat team also arrived to inspect the lavatory, according to WUSA.

BWI updated its Twitter page shortly after 9:30 p.m. to announce that authorities determined the substance to pose no threat.

“Fire and Rescue Department @BWIFire and other authorities investigated unknown substance on board an aircraft that had arrived at airport. Testing indicated there was no hazard. The scene was secured,” the tweet read.

Southwest Airlines confirmed to Fox News that the plane was “cleared to return to service.”

“Flight #239 with scheduled service from San Antonio to Baltimore/Washington landed safely without incident as scheduled in Baltimore on Monday evening, where local authorities were called to meet the flight out of an abundance of caution due to an unknown substance found in one of the lavatories,” a representative for the airline stated.

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“Local authorities tested and identified the substance as paper fibers from a tissue box, and the aircraft has been cleared to return to service.”