Investigators say they do not know the motive for Wednesday's bombing in the southern Philippines that killed two Philippine citizens and one American Army sergeant.

The mayor of Zamboanga, Maria Clara Lobregat, says the Philippine military, the national police and the U.S. Federal Bureau of Investigation are examining the bomb scene. "The bomb was placed in a motorcycle and parked in front of the house, for the bomb to explode right there," he said.

The owners of the house ran a small bar and restaurant frequented by soldiers from a nearby Philippine military camp.

Philippine military officials confirmed Thursday that two civilians were killed, along with the U.S. soldier, and 23 people were injured in the blast.

About 250 U.S. soldiers are in the southern Philippines. They are the last of more than 1,000 Americans who trained with Philippine soldiers for six months in an exercise aimed at wiping out the Abu Sayyaf, an extremist group that claims to be trying to create a Muslim state in the Philippines.