Colombo launched a full-scale investigation on the bomb hoax which triggered an emergency evacuation from the Riyadh/Jeddah bound flight Thursday afternoon. The grounded aircraft also left Colombo at 8 p.m. yesterday without passengers on board after airport authorities issued a clearance certificate for its takeoff. "We are trying to trace the call which initially came to the airport telephone operator and then connected to the control tower," airport chief Tiran Alles said. "An investigation is under way."Nineteen passengers who were seriously injured in the scramble to exit the Boeing 747 jetliner through emergency chutes were still in hospital yesterday, a day after the incident. A total of 62 out of the 430 passengers and 22 crew members were injured and most of them had minor bruises. Alles said they were probing the incident thoroughly. "We have had similar hoaxes in the past," Alles said. "This is serious because a life has been lost and many more people were injured."