Authorities later discovered that a passenger on the aircraft's preceding leg was Ramzi Yousef. He was later convicted of the 1993 World Trade Center bombing. Yousef boarded the flight under the fake Italian name "Armaldo Forlani", a wrong spelling of the name of the Italian legislator Arnaldo Forlani.
Yousef assembled a bomb in the lavatory and put it in the lifejacket pocket under Seat 26K on the right-hand side of the fuselage, setting the timer to detonate the explosives four hours later. Domestic flight attendant Maria Delacruz noticed that Yousef kept switching seats during the course of the Manila to Cebu flight. Yousef and 25 other passengers left the plane at Cebu. 256 passengers and a different cabin crew boarded in Cebu. Many of them consisted of Japanese people; some of them were coworkers traveling as part of a tour group. Airport congestion delayed the departure of Flight 434 from Cebu for 38 minutes. All of the passengers boarded by 8:30 a.m.; the bomb had been planted around two hours earlier. PAL 434 cleared for takeoff at 8:48 a.m.
Two hours before arrival at Tokyo, the bomb exploded at 11:43 a.m. while Flight 434 cruised on autopilot 31,000 feet (9,400 m) above Minami Daito Island, which is located near Okinawa Island and is 260 miles (420 km) southwest of Tokyo.
The explosion ripped the body of 24-year old Haruki Ikegami (池上春樹, Ikegami Haruki ), a Japanese businessman occupying the seat, in half. He was an industrial sewing machine maker returning from a trip to Cebu. The lower half of his body fell into the cargo hold. Ten passengers sitting in the seats in front of and behind Ikegami were also injured; one needed urgent medical care. The bomb tore out a two square foot (0.2 m2) portion of the cabin floor, revealing the cargo hold underneath, but the fuselage of the plane stayed intact.