Deputies are investigating a bomb threat that forced about 170 passengers to evacuate a flight leaving from St. Pete-Clearwater International Airport (PIE) on Friday evening. Pinellas County Sheriff Bob Gualtieri said the Allegiant Air flight from Cincinnati had just landed at the airport and was set to turn around quickly and return to the city. But after the passengers boarded the flight and the plane had taxied away from the gate onto the runway, a flight attendant decided to open the bathroom door. The flight attendant saw a "very explicit" bomb threat written on the back of the bathroom door, the sheriff said. They immediately told the pilot, who stopped the plane and evacuated the passengers. Gualtieri did not give details on what was written but said it affected all Allegiant planes at the airport. The sheriff's office responded with bomb dogs that searched at least six Allegiant planes. "We found no evidence of any explosive devices," the sheriff said. He confirmed that there is "no immediate threat" and "no evidence that anybody actually did anything" in terms of planting explosives. No one is in custody, but the sheriff said there's a person investigators are focusing in on. “Their actions were suspicious and we’re looking into it," Gualtieri said. The Federal Aviation Administration confirmed that the airport was closed for several hours but has since reopened. “To say it’s a disruption and a mess is an understatement," the sheriff said. The investigation is ongoing.