Wahyudi Soeriaatmadja – A terrorist group behind last month's suicide attack at a police mosque in Cirebon, West Java, had planned at least six similar attacks elsewhere, Indonesian police said on Thursday.

Six bombs similar to the one used in the mosque attack were retrieved on Wednesday from the bottom of a river in Cirebon, 200 kilometers southeast of Jakarta. A key member of the terrorist group confessed to throwing them there in a panic once the group knew police were onto them.

The arrests come ahead of a big regional meeting in Jakarta this weekend. Hundreds of foreign delegates are expected to attend the 18th Asean summit tomorrow and on Sunday. Jakarta is now on high alert, with three bomb scares in the city on Wednesday alone.

Last month, Muhammad Syarif, 32, blew himself up during Friday prayers, killing himself and injuring about 30 worshippers, all but two of them policemen. The bomb carried by Syarif contained, among other things, potassium nitrate, calcium chloride (a raw material used to make toothpaste), and shrapnel such as nails.

Police have arrested four people believed to be linked to Syarif, including a key figure known by the initial M, who might have played an important role in procuring the explosives and making the bombs.

Syarif's younger brother Muhammad Basuki, alleged to have been intended as the group's next bomber, has also been arrested.

Police arrested M in Slawi, Central Java, late on Monday. They seized a grenade, jihadist books and nails. A series of interrogations of M led the police to the Soka river in Cirebon. Local television stations showed footage of a bomb squad going into the river to search for and remove the soaked pipe bombs, which could no longer be used.

"We won't stop our investigation here. We want to know who else is in this group, and whether there is any financier," National Police spokesman Boy Rafli Amar told reporters on Thursday. Police are also trying to pinpoint the would-be suicide bombers as well as their targets.

Terrorism analyst Mardigu Wowiek Prasantyo said militants are learning from each other in media reports. That is why, he said, different militant groups may apply the same methods, even if they are not affiliated.

"Syarif's cell used raw material to make toothpaste to make bombs. That was just some common whitening material that was easy to get here. This was then reported publicly, and unfortunately became a lesson to other militant groups, who may in turn use the same material next time," Mardigu told The Straits Times by telephone.

On April 21, five massive bombs near a Catholic church outside Jakarta, set to detonate during Easter celebrations, were defused by a police squad. The bombs, weighing a total of 150 kilograms, were placed near a gas pipeline that extends beneath the church and around houses at a nearby residential complex.

The mastermind of that attack, former journalist Pepi Fernando, in his early 30s, was also behind a series of so-called book bombs in March that were sent to, among other people, a moderate Muslim leader and an anti-terrorist veteran policeman.

On Wednesday, several suspicious objects – a fuse inside a plastic pipe and boxes on the seat of a parked car – were spotted by a security guard, who alerted the police. The car was parked near an entrance to the convention center where the Asean summit will be held. Further checks showed the objects were not bombs.