Investigations are underway after a hoax bomb warning was found aboard a Turkish Airlines (THY) jet that arrived in Istanbul from the Estonian capital, Tallinn.Turkey's national flag carrier announced on Thursday that flight TK 1422 landed at Istanbul's Atatürk International Airport at 7:15 p.m. on Wednesday. After passengers had left the aircraft, cleaners found a note written in English that claimed there was a bomb on the plane. Turkish police and bomb-disposal experts searched the whole aircraft before declaring it to be a false alarm.One passenger, an 18-year-old foreign national, was taken to the airport's police station for questioning.The false alarm comes three days after a THY flight attendant was detained in connection with a recent bomb scare on an international flight. The flight attendant has been accused of leaving a note warning of a bomb aboard a THY flight. Turkish Airlines confirmed Monday that the unidentified employee had been sacked after being held by police.Wednesday's incident is the latest in a series of hoax bomb threats issued against the airline's operations that have resulted in the emergency landing of at least four international flights since the end of March.