A gunman shot dead nine Philippine soldiers in a rampage at a military base on the troubled island of Jolo before he was also killed, sources said yesterday.
It was not clear whether the shooter was a soldier or a member of a pro-government militia helping troops in the hunt against al-Qaeda-linked Islamic militants on Jolo, they said.
The man ran amok and started firing indiscriminately some time on Saturday morning before he was killed by troops, the sources said.
Military officials declined to issue an official statement, but sources confirmed the incident was now under investigation.
More than 5,000 troops backed by US advisers are on Jolo island to track down Abu Sayyaf militants and several key leaders of Jemaah Islamiyah, militant groups which have been blamed for a number of terror atrocities.
Lieutenant Colonel Ernesto Torres said the army was investigating whether the civilian was behind the attack.
"The slain unidentified civilian might have an involvement. As of the moment, we are investigating it. We are looking into possibilities. Why was there an unidentified civilian in the camp?" Torres said.
"There was a firefight, an exchange of gunfire. When the battalion commander reinforced the area after it was reported to him, they were also fired upon," Torres said, adding that the camp was without electricity during the attack.
Since August, nearly 10,000 soldiers have been battling the Philippines' deadliest Muslim rebel group, the Abu Sayyaf, in fierce clashes on Jolo, a kidney-shaped island around 950km south of Manila.
The ground offensive has been the most successful yet against Abu Sayyaf. Around 70 militants, including the group's top two leaders, have been killed out of a 400-strong outfit.
Dozens of troops have also been killed and more than 100 wounded in the fighting, which has also displaced thousands of villagers.
Abu Sayyaf is the most militant of four Islamic rebel groups in the Philippines, a largely Catholic country, where Muslims in the south have been seeking independence for decades in a conflict that has killed more than 120,000 people.