SYDNEY, Dec. 18 (Xinhua) -- A Tigerair flight from Sydney to Melbourne was forced to turn around on Monday evening mid route, after a "onboard incident."

Although it is still not clear what the "incident" was, local media have reported that crew members found a "written threat" on board flight TT271.

According to the airline, "the Captain made the decision to return to Sydney following an incident on-board, in line with standard procedures," Tigerair told the Australian Associated Press.

"The safety of our crew and guests is always our highest priority and we are working with the relevant authorities to investigate the matter."

When the plane landed on the tarmac, passengers were made to switch their phones off while they waited for two hours for federal police officers to clear the plane.

"All guests and crew safely disembarked the aircraft and have been re-accommodated on the next available flight," Tigerair said.

"We sincerely apologize for any inconvenience caused, however the safety of our crew and guests is always our highest priority," Tigerair said.