Four workers were hospitalized as a precaution after a letter containing a suspicious substance was delivered to the Federal Aviation Administration's (FAA) Twin Cities offices.
Employees were evacuated from the office near Minneapolis-St. Paul International Airport on Tuesday following the discovery, with several seeking medical help at Hennepin County Medical Center.
The FBI, with help from the Department of Homeland Security, has launched an investigation into the letter and is testing its contents.
The FAA building is located at 6020 South 28th Ave., at the north end of the sprawling MSP Airport area, near the Metropolitan Airports Commission offices, and is completely separate from passenger terminals.
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Those employed at the office oversee aircraft certification, security, pilot licensure and regulatory compliance, as well as airline and airport regulatory compliance, the FAA told Bring Me The News in a statement.
Air traffic control operations are not handled in the building and were not impacted by the evacuation.