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Landover, Md. (WUSA9) -- Prince George's County Hazardous Materials teams and federal investigators descended on a small warehouse in Landover Wednesday after screening machinery there detected a "positive hit" in a piece of mail headed to federal offices.

Prince George's County Fire Chief Marc Bashoor described the response at the warehouse operated by contractor Bright Key as "an abundance of caution". Bashoor said the "positive hit" had not been confirmed. He did not say what substance may have been detected.

No workers or responders have been injured, Bashoor said.

Bashoor said machinery detected possible contamination in mail at 6:30 a.m. Hazardous materials workers and investigators remained on the scene 12 hours later.

Bashoor did not know who the mail was addressed to. He referred questions about the investigation to the FBI, which has not released any information about the incident.

Bright Key is a contractor that handles mail before it is delivered to federal offices, according to online records of federal contracts.

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