A Philippine court on Friday found 66 alleged members of the Abu Sayyaf guilty of kidnapping dozens of students, teachers and a Catholic priest in the south in 2000, in the largest single conviction involving the brutal Muslim militant group.

The Regional Trial Court branch 261 acquitted for lack of evidence 20 other people who have languished in jail for a number of years while insisting they were innocent in the brazen March 2000 kidnappings of 52 people, mostly young students at two schools on Basilan island.

Two kidnapped teachers were beheaded and a priest died while in Abu Sayyaf custody. The other hostages were rescued or freed after local officials negotiated for their release a few days after they were kidnapped en masse from the schools in the villages of Tumahubong and Sinangkapan.

The Abu Sayyaf — or Bearer of the Sword — has been listed as a terrorist group by the U.S. It was founded around the early 1990s in poor, predominantly Muslim Basilan province. An unwieldy collection of Islamic preachers and outlaws, it vowed to wage jihad, or holy war, but lost its key leaders early in combat, sending it on a violent path of extremism and criminality.

One of its top commanders, Isnilon Hapilon, helped lead a siege of southern Marawi city by Islamic State group-aligned militants in May last year which left more than 1,000 people dead, mostly militants, including Hapilon. Hundreds of thousands of villagers were displaced. Troops backed by U.S. and Australian spy planes quelled the siege after five months.

Today, the Abu Sayyaf has degenerated into a few loose factions with a few hundred ragtag fighters and no central leader in Basilan and outlying islands. But it remains resilient and violent, engaging in ransom kidnappings and extortion that have allowed it to survive without extensive backing from foreign extremist groups.

Senior State Prosecutor Peter Ong said some innocent suspects were arrested but only two were freed by the court a few years ago after he and other prosecutors took steps to secure their release before the trial ended.

"The persistence and determination of the court to give justice to the victims and those who were wrongfully arrested led to this decision, although it took more than one and a half decades to happen," said Ong, one of several prosecutors who handled the case.

Nearly 100 people were charged in the Basilan kidnappings. An Associated Press investigation in 2014 that included interviews with prosecutors and key witnesses showed dozens of people were detained despite a lack of evidence against them.

An Abu Sayyaf commander, Abu Gandhie, who took part in the planning but was captured and later turned state witness, told the AP at the time that more than 40 militants took part in the mass kidnappings, including more than 10 who were later killed in clashes with troops. When he testified in court, he identified only 12 of the dozens of men who were charged as among those who actually took part in the abductions.

Mistakes are a concern in the Philippines' slow and overburdened law enforcement and criminal justice system, which has a backlog of thousands of cases and is tainted by corruption allegations.

In far-flung Muslim regions in the south, those frailties are compounded by conditions such as a lack of birth certificates and other identification papers for poor villagers, hampering the accurate identification of suspects. With spotty intelligence, government forces have often relied on civilian informants, some with questionable backgrounds, human rights advocates say.