A bomb planted by the brutal Abu Sayyaf (search) group caused a ferry fire in February that killed over 100 people in the Philippines' worst terrorist attack, an investigation concluded Monday.

President Gloria Macapagal, who initially downplayed a claim of responsibility by the Al Qaeda-linked (search) Abu Sayyaf, said six people, including two arrested shortly after the attack, have been charged. am of Wichita, Kan. also died during a rescue mission that freed his wife, Gracia.

"I am now instructing the police and the military to intensify the manhunt for the two masterminds — Khaddafy Janjalani (search) and Abu Sulaiman — and their two other accomplices," Arroyo told a news conference.

Sulaiman was the target of an unsuccessful U.S.-backed operation two weeks ago aimed at capturing or killing him, his son and other Abu Sayyaf members. He and Janjalani, the group's main leader, already had $5 million bounties on their heads.

The Al Qaeda-linked Abu Sayyaf, which is on Washington's list of international terror groups, claimed responsibility immediately after the Feb. 27 fire aboard the Superferry 14, saying it planted a bomb inside a television set that one of its members carried aboard.

A blast and fire struck the ferry, carrying about 900 people, an hour after it left Manila for the central and southern Philippines. The official report said the bodies of 63 people were recovered and that 53 others are missing and believed dead.

The overall presumed death toll of 116 would make it Southeast Asia's second-worst terror attack after the 2002 Bali bombings that killed 202 people on the Indonesian resort island.

Transport Secretary Leandro Mendoza said investigators believe the Superferry 14 was targeted because owners WG&A had not complied with an Abu Sayyaf letter last year demanding protection money.

A preliminary investigation had indicated two possible causes — a bomb or a gas explosion.

Mendoza said the investigation dragged on so long because it took five months to right the ferry, which ended up lying on its side in shallow water, and investigators then encountered a tangle of twisted metal that supported the conclusion that a bomb caused the disaster.

In March, Arroyo announced the arrests of six Abu Sayyaf members, including one who allegedly confessed to planting the bomb, and the discovery of a cache of explosives that police said was to be used to bomb malls, trains, an oil depot, foreign embassies and other targets in Manila.

Security officials have said Redendo Cain Dellosa confessed after his arrest that he stashed about 8 pounds of TNT in a TV set that he carried onto the ferry. Dellosa later claimed he was tortured into signing a confession.