Here is a brief look at Jemaah Islamiyah whose alleged leader, Abu Dujana, was arrested over the weekend.

Organization: Its name means "Islamic Community." The loose network of around 1,000 Islamic militants extending across Southeast Asia was formed in the 1990s in Malaysia. It has been severely weakened by police crackdowns in Indonesia, Malaysia, Singapore and the Philippines since 2001. Leaders are mostly Indonesian nationals who fought or trained in Afghanistan in the 1980s and 1990s. It was once known to have various territorial divisions called "mantiqis" that covered the region. Intelligence officials say some operations have been financed by Osama bin Laden's al-Qaida.

Activities: Initially formed to establish an Islamic state across much of Southeast Asia, some of the hard-line members fell under the influence of al-Qaida in the early 2000s and launched a terror campaign, reportedly against the wishes of its leaders who saw such tactics as counterproductive. The split remains until today, with a hard-line minority that supports attacks against civilian, or "soft" targets, and more moderate members who do not. Its members have been blamed for the 2002 bombings on the resort island of Bali, the 2003 and 2004 attacks on the J.W. Marriott Hotel and the Australian Embassy, and the 2005 triple suicide bombings at restaurants in Bali. Together, the attacks killed more than 240 people, many of them Western tourists. Some 300 have been convicted in the bombings and related terrorist offenses.

Leaders: The group's founders are alleged to be Indonesians Abdullah Sungkar, who died in 1999, and Abu Bakar Bashir, who was released from prison last year and has since returned to preaching. One of their first recruits was Riduan Isamuddin Hambali, an alleged top al-Qaida operative now in US custody for his alleged links to the Sept. 11 terror attacks in the United States. Another top member, Azahari bin Husin, was killed last year in a shootout on Java island as he detonated his suicide belt. Indonesian officials say the group's leaders now include Indonesians Zulkarnaen, Dulmatin and Umar Patek. Police say one-time member Noordin Top, a Malaysian seen by most analysts as the most dangerous terrorist in Indonesia, now heads a breakaway faction.