Jakarta's governor has urged residents to be on guard against terror attacks as Indonesia heads towards next month's elections.
The warning came as police continued to question a group of Muslim radicals arrested after they accidently caused an explosion in their house in a Jakarta suburb during a bomb-making class.
"The incident in Depok should be a warning that there are still terrorists around us and this cannot be handled by the [security] apparatus alone," Governor Sutiyoso told reporters after a meeting at city police headquarters. "There is a need to be on alert against terror actions that are done to scuttle the elections."
A blast during a bomb-making class on Sunday blew the roof off a house in Depok on the southern outskirts of Jakarta. Police detained at least nine suspects and say they plan to charge them under the country's anti-terror law.
ElShinta news radio said three more people, including a woman, have now been detained in addition to the nine. Police could not be reached for comment.
Police said one device exploded accidentally as a group of men were practising bomb-making at the back of the house while at the front, their wives were engaged in a Koranic recital.
Jakarta police chief, Inspector General Makbul Padmanegara, said the group was preparing bombs to battle enemies of Islam.
Nine metal tubes, parts of a detonator and timer, and potassium chlorate and sulphur – the same chemicals used for the Bali bombings – were found at the scene.
Indonesia has suffered a spate of bombings, the most recent of which have been blamed on the Al-Qaeda-linked Jemaah Islamiyah (JI), including the Bali nightclub attack which killed 202 people in October 2002 and the Marriott hotel bombing last August which killed 12 people The US State Department last Friday warned of possible further attacks linked to the April 5 general election.
Padmanegara has said police could not yet tie the nine people arrested this week to any particular group. But he said Monday that one of the suspects, Oman Rahman, had cautioned his relatives against visiting "dangerous" shopping malls in a letter confiscated by police.
Rahman also urged them not to take part in the general election which he called an "act that runs against the religion." Several newspapers criticised police for failing to detect the militant group earlier.
Padmanegara said its members in recent months conducted physical exercises and practised martial arts in fields in the area.
Local people interviewed by Tuesday's Jakarta Post said group's members, while polite, had little to do with their neighbours. They christened them "the black veil group" after the all-enveloping garments worn by the women.
Sutiyoso said isolated houses, or those with occupants who avoided their neighbours, could be used by terrorists. He said the people's participation, as the eyes and ears of the security forces, was important in fighting terrorism. "We cannot loosen our vigilance."