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Pentagon Spokesman Bryan Whitman says the incident is under investigation, but early indications are that the troops' vehicle was targeted by a roadside bomb similar to the ones used by insurgents in Iraq and Afghanistan.
Whitman says the two Americans and one Filipino killed on Tuesday were on their way to re-supply troops helping to build a school on the southern island of Jolo.
While this is the first attack on U.S. troops in the Philippines in seven years, Whitman says they are aware of the constant danger.
"The threat has been well known," Whitman said. "They operate cognizant of the threat that is there. The threat didn't diminish or necessarily increase as a result of this particular incident. But this is the first time, at least in any recent history that I can recall, that a particular device of this nature was used against U.S. forces. So obviously, the command is going to be taking a look at that."
It was 2002 when the last American service member was killed in the Philippines by a bomb. That incident occurred when explosives attached to a motorcycle detonated on a different southern island.
The Philippine military says it suspects the Abu Sayyaf group was responsible for Tuesday's attack. Abu Sayyaf is a militant Islamic separatist group with ties to al-Qaida terrorists, who are fighting U.S. efforts in Iraq and Afghanistan, as well as conducting attacks around the world.
About 600 U.S. troops are in the Philippines, training and advising the Philippine military and helping with humanitarian projects. The training focuses on counterinsurgency tactics, through force and through civic projects, to help the Philippine government combat Abu Sayyaf and other insurgent groups.