ANKARA, Turkey -- Turkish Airlines says a Basel, Switzerland-bound flight returned to Istanbul after a note saying there was a bomb on board was found in a toilet.

The airline said Friday that Flight TK1923, with 151 people on board, headed back to Istanbul to be searched. The threat turned out to be a hoax.

The passengers are being flown to Basel on another plane.

It was the third such incident in recent weeks. On March 30, the company diverted an Istanbul-to-Sao Paulo flight to Morocco after finding a note with the word "bomb" in the toilet. No bomb was found. A day earlier, a flight to Tokyo also returned to Istanbul before being cleared.

Turkish Airlines says it is standard procedure to make emergency landings under such circumstances.