The backpack bomb that killed at least 20 people and injured scores of others in the southern Philippines Tuesday could be the handiwork of any of the three rebel groups — the Moro Islamic Liberation Front, the Abu Sayyaf, and the New People's Army — currently operating in the area.

No one has yet claimed responsibility for the blast at the Davao City International Airport on the main southern island of Mindanao. But the military has blamed the Moro Islamic Liberation Front for a string of recent attacks, including a car bombing at nearby Cotabato airport last month that killed a woman. The MILF denied involvement in Tuesday's attack.

The southern Philippine islands are relatively poor and have a large Muslim population — called "Moros," Spanish for "Moors" — in comparison to the rest of this overwhelmingly Roman Catholic country. The central government has been trying to encourage economic development in the area, but Manila's efforts to crack down on militants have been undermined by weak and sometimes corrupt law enforcement.

The first major Muslim rebel group, the Moro National Liberation Front, was founded around 1970 and originally demanded an independent Muslim state in the southern islands. At one time, it enjoyed the support of major Muslim countries such as Libya, Saudi Arabia, and Malaysia.

The MNLF's goals eventually shifted to autonomy within the Philippines, and it reached a peace agreement with the Philippine government in 1996, resulting in the establishment of the Autonomous Region of Muslim Mindanao.

Nur Misuari took the governorship of the four-province region but, his rule ended in November 2001 after he led a failed uprising. Another MNLF leader, Parouk Hussin, took over.

The similarly named Moro Islamic Liberation Front split off from the MNLF in the late 1970s. It has signed at least two cease-fires, the more recent with the current administration of President Gloria Macapagal Arroyo in August 2001, but fighting sometimes flares up.

The MILF still has several thousand fighters on Mindanao, and intelligence officials say Al Qaeda may have cultivated ties to the group in the 1990s.

Muhammad Jamal Khalifa, a Saudi expatriate and a brother-in-law of Usama bin Laden, allegedly has supplied money to the MILF as well as to the smaller Islamic group still fighting, Abu Sayyaf.

Hambali, a senior Indonesian Islamic militant believed to have been instrumental in the Bali nightclub bombing of Oct. 2002, may also have links to the MILF, according to the Council on Foreign Relations.

In January, Philippine officials arrested three suspected MILF members for helping an Indonesian with suspected Al Qaeda ties bury a cache of weapons and explosives.

The United States does not consider the MNLF or the MILF to be terrorist organizations.

The smallest and most radical Islamist group, the Abu Sayyaf — "Bearer of the Sword" in Arabic — has about 200 fighters and enjoys no support from foreign states or the larger Muslim groups, is considered by the U.S. government to have Al Qaeda ties.

Abu Sayyaf broke off from the MNLF in 1991. Its first leader, Abdurajak Janjalani, fought in Afghanistan during the Soviet occupation, but was killed in firefight with Philippines police in 1998.

Abu Sayyaf's pan-Islamic goals, radical ideals and bandit-like tactics — which include the abduction of women — differentiate it from the MNLF and MILF, which distance themselves from Abu Sayyaf.

The group is considered by the U.S. government to have Al Qaeda ties, but evidence linking the two organizations since 1996 has not been made public. Khalifa, bin Laden's brother-in-law, reportedly provided financing and support for Abu Sayyaf in its infancy.

Abu Sayyaf finances itself largely through kidnapping and extortion. Its most audacious act was the April 2000 abduction of 21 people, including 10 foreign tourists, from a resort area in Malaysia. The government of Libya later secured the release of the hostages with millions of dollars in ransom payments.

In June 2002, U.S.-trained Philippine commandos tried to rescue three other hostages being held by Abu Sayyaf on Basilan Island in the southern Philippines. Two hostages — a Filipina nurse and an American missionary — were killed in the shootout, but another American missionary, Gracia Burnham, was freed.

The American government is so strongly convinced of Abu Sayyaf's Al Qaeda ties that U.S. soldiers returned to the southern Philippines last year in the first operations in the war on terrorism outside of Afghanistan.

About 1,200 troops, including 160 special forces, were sent to "train, advise and assist" Filipino forces battling Abu Sayyaf on the island of Basilan, and in February of 2003, more than 1,000 U.S. troops prepared to be deployed to the island of Jolo.

The Jolo offensive was put on hold after Pentagon officials admitted Americans might be drawn into battle. The Philippines' constitution bars foreign troops from combat in the former U.S. colony, and negotiations between Manila and Washington about the American troops' roles continue.

In addition to the Muslim groups, the communist New People's Army has been fighting a guerrilla war against the government on Mindanao for over 30 years.

The NPA ended peace talks with the government almost three years ago, and there are some indications it has forged a tactical alliance with either or both the MILF and Abu Sayyaf.

In December 2001, the State Department added the NPA to a list of terrorist groups whose members should be denied U.S. visas.

But the group, which attacked U.S. military personnel and installations before the closure of U.S. bases in the Philippines in 1992, is not on the Bush administration's main post-Sept. 11 list of terrorist organizations, and its assets have not been frozen.

In addition to the Muslim guerrilla and terrorist activity in the southern islands, Manila was home to a particularly active Al Qaeda cell, consisting largely of foreigners, in the mid-1990s.

Khalid Shaikh Mohammed, captured in Pakistan this weekend and suspected of being the mastermind of the Sept. 11 terror attacks, was part of the Manila cell, as was his nephew, Ramzi Yousef, who was convicted of planning the 1993 bombing of the World Trade Center. Both men are Pakistanis.

The Manila cell reportedly planned to assassinate Pope John Paul II when he visited the Philippines in 1995, and also plotted to blow up 11 American airliners over the Pacific simultaneously.

Both plots were foiled when police operatives arrested a cell member, then found bomb-making ingredients as well as plans for the aforementioned attacks during a search of Yousef's Manila apartment.

The Al Qaeda cell did carry out several successful bombings in the Manila area in 1994, with targets including the Miss Universe beauty pageant, Wendy's hamburger restaurants and a movie theater.

Philippines Police Director Avelino Razon, whose testimony convicted Ramzi Yousef, strongly believes the recent string of bombings in Mindanao are the handiwork of terrorists who have links to Al Qaeda.

He cites an explosion that toppled high-tension power line towers, attacks on police checkpoints and a municipal town hall and an explosion last month that killed one and injured six in Cotabato City.

While most members of the Al Qaeda cell that operated in the Philippines have been arrested, Razon believes there are still members of the group capable of launching a terror attack.

Fox News' Mike Cohen and The Associated Press contributed to this report.