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Officials say tert-Butyl hydroperoxide was to blame for a HAZMAT scare at the post office at Tampa International Airport Wednesday.

Tampa Fire Rescue says the chemical is used to make "various commercial and industrial products." Officials said a container inside a shipping box leaked the chemical, causing postal employees to complain of burning eyes and difficulty breathing from what they called a "vinegar or bleach smell”.

According to Tampa Fire Rescue, nine people required medical treatment; four people were treated at the scene and five others were taken to St. Joseph’s Hospital with minor injuries.

The postal facility remained evacuated for hours while the Tampa Police Department bomb squad and other agencies investigated the suspicious smell.

Tampa Fire Rescue said the leaking container was "clearly marked with a label indicating its contents," but it was unclear if the shipping box was labeled.

The airport post office, which is one of the city’s largest postal facilities, was reopened Wednesday afternoon.

It is on the grounds of Tampa International Airport but is not physically connected to any airport passenger facilities. Airport operations were not affected by the incident.