The Jemaah Islamiah network, whom south-east Asian terrorism experts blame for the Jakarta bomb, is seen as the regional arm of Osama bin Laden's al-Qaeda.
The Jemaah Islamiah (Islamic Community), once little known outside south-east Asia, was blamed for the 2002 bomb attacks in Bali that killed 202 people, and for a car bombing of Jakarta's J.W. Marriott hotel last year that killed 12 people.
Jemaah Islamiah's avowed aim is to establish a conservative Islamic state across the Malay archipelago encompassing Indonesia, the southern Philippines, Brunei, Singapore, peninsular Malaysia and southern Thailand.
The group has its roots in Indonesian Muslim nationalist movements of the late 1940s and early 1950s. Most of these were crushed or faded away in the 1960s as independent Indonesia's second president, Suharto, took power.
Abu Bakar Bashir, the 65-year-old cleric described as the group's spiritual leader, is currently in jail in Indonesia awaiting terror-related charges. Bashir, of Yemeni descent, became active in the 1970s, leading an Islamist youth movement in Indonesia under another cleric, Abdullah Sungkar.
In 1985, a string of bombings, fires and unrest around Jakarta prompted Suharto to order a new crackdown on the Islamist opposition. Sungkar and Bashir fled to Malaysia.
As Suharto's rule ended in 1998, Bashir, at the centre of a network that security officials say was radicalised by contacts with al-Qaeda, returned home.
After September 11th, Jemaah Islamiah switched its attention to Western targets in south-east Asia.