ZAMBOANGA, Philippines (AP) — A Philippine air force helicopter on its way to airlift troops wounded in a recent suicide bombing crashed on a southern island Wednesday, killing all four crew members on board, officials said.

The Sikorsky S-76A took off from southern Zamboanga city for a flight to nearby Sulu province and crashed midway in rainy weather in the hilly village of Upper Manggas on Basilan island, the air force and police said.

The two pilots and two other crew members were killed. They were bound for Jolo town to airlift troops who were wounded in two Aug. 24 bombings blamed on Abu Sayyaf militants, air force spokesman Lt. Col. Aristides Galang said.

The bombings killed 15 people, including soldiers, along with two suspected female militants, and wounded more than 70 soldiers, policemen and civilians in the worst Muslim militant attacks in the country this year.

Officials were investigating the cause of the crash. Other Sikorsky helicopters were grounded for safety checks while the investigation is underway, the air force said.