Four agents of the U.S. Federal Bureau of Investigation (FBI) have arrived in Manila to build a strong legal case against the Abu Sayyaf, which is currently holding hostage two Americans and a Filipina nurse in the jungles of southern Basilan, the authorities said yesterday.

The FBI agents, whose names were not disclosed, met Philippine National Police (PNP) Director General Leandro Mendoza, on Monday, a PNP source said.

According to the source, who requested anonymity, the U.S. agents are in the Philippines to gather evidence against the Abu Sayyaf, which also publicly admitted the execution of an American hostage, Guillermo Sobero, in Basilan last year.

In a statement made late in June, Abu Sayyaf spokesman Abu Sabaya said his group had beheaded Sobero as an Independence Day gift for President Gloria Macapagal-Arroyo. The Philippine National Day falls on June 12, the same day Sobero is believed to have been executed last year.

Sobero was abducted along with American couple Martin and Gracia Burnham as well as 17 other Filipinos by an Abu Sayyaf group led by Sabaya last May 27 in western Palawan island. Most of the hostages have either been released or executed while the Burhams and Filipina nurse Deborah Yap remain in the hands of the extremists.

Sobero's decapitated remains were exhumed late last year from a grave deep in the jungles of Basilan and were turned over to the U.S. authorities. "The FBI agents are here to build a strong case against the Abu Sayyaf so that they can file a case against them in the courts of the United States."

This will be similar to what they filed against Osama bin Laden and others who are believed to be responsible for the September 11 atrocity. Although top Abu Sayyaf leaders such as Sabaya are facing criminal cases in local courts, the authorities have so far been unable to capture any high ranking members of the group except for Nadjmi Saabdullah, who is currently languishing in prison following his arrest last year in southern General Santos City.

Saabdullah is believed to be the ideological leader of an Abu Sayyaf faction based in southern Sulu. The Abu Sayyaf in Sulu are being held responsible for the abduction of 21 people - mostly foreigners - in Sipadan island, located across the border in Malaysia.

The source also said that the presence of the FBI agents in Manila is not a coincidence with the arrival of U.S. troops in Basilan, fuelling speculations that the American presence may be aimed at direct involvement in the Philippine government's fight against the extremists.

Advance elements of the U.S. military's 160-strong contingent have already arrived in Zamboanga City, some 25 km from Basilan, in the first phase of six-month long military exercises with local troops to be conducted right at the heart of the conflict area in the tiny southern island.

Both the U.S. and Philippine governments have stressed that the arrival of American soldiers is for training purposes only. There have been suspicions that the foreign troops will conduct a rescue operation for the remaining Abu Sayyaf hostages.

The source said that if the FBI is successful in building a strong case against Abu Sayyaf leaders and subsequently files a case against them in U.S. courts, there is a possibility that members of the group - once they are captured - will be sent to the United States for trial.