LOMBOK, Indonesia — The Indonesian police say they have foiled a suspected terrorist cell with the ability to use Wi-Fi to detonate explosive devices, highlighting advances in bomb-making in a country with a history of militant activity tied to the Islamic State.
Several of the suspects, who were arrested in raids last week on the densely populated island of Java, are members of Jamaah Ansharut Daulah, or J.A.D., a local militant group that has pledged allegiance to the Islamic State, the police said.
During the raids, counterterrorism agents found bomb-making equipment and traces of triacetone triperoxide, or TATP, a highly unstable homemade explosive that is sometimes used by the Islamic State outside the Middle East. TATP was used in Islamic State bombings in Paris and Brussels, as well as last month in Sri Lanka, where more than 250 people were killed by suicide attacks at churches and luxury hotels.
One of the suspects in the Indonesia plot, a skilled bomb-maker who was arrested on May 8, was perfecting the process of detonating a bomb through Wi-Fi networks, Dedi Prasetyo, the national police spokesman, said on Thursday. Mr. Dedi said that militants were planning to launch attacks on Wednesday, when the official results of Indonesia’s national elections are expected to be tallied.
Because Indonesia has been the site of repeated attacks by radical Muslim militants, the authorities sometimes use phone-signal jammers during mass gatherings to prevent bombers from remotely activating explosive devices .
But Mr. Dedi said that jamming a Wi-Fi signal is harder to do. By using a router to strengthen a Wi-Fi signal, a bomber could theoretically extend the signal range from 200 meters to 500 meters, he said. With an amplifier, that range could extend as far as a kilometer, or about two-thirds of a mile.
“One guy with electronics skills, that’s all you need,” said Sidney Jones, the director of the Institute for Policy Analysis of Conflict in Jakarta and an expert on Southeast Asian militant groups.
On Tuesday, counterterrorism officers on Java arrested nine people suspected of militant activity, seven of whom had returned from Syria, where the Islamic State had constructed a caliphate before its territory was overrun earlier this year. One of those arrested who had spent years in Syria held a prominent position with the Islamic State, the police said.
“We are still questioning them since they were just arrested two days ago,” Mr. Dedi said. “The network, the structure of the organization, the plans.”
Hundreds of Indonesians went to fight with the Islamic State, but critics of the government’s counterterrorism strategy say the fighters have avoided the radar of local law enforcement upon returning home from the battlefields of the Middle East.
“Local authorities have limitations in executing monitoring because of budget constraints,” said Alto Labetubun, an Indonesian terrorism analyst who has worked in Afghanistan and the Middle East. “They do register returnees — but imagine the manpower needed if you want to do surveillance on them after they return to their hometowns.”
Even though the Islamic State, also known as ISIS, lost its territory in Iraq and Syria, the terrorist organization has been able to metastasize far from its former self-proclaimed caliphate, spreading its tentacles throughout Southeast Asia and elsewhere. In January, a group that had sworn loyalty to the Islamic State bombed a cathedral in the southern Philippines. And the attacks in Sri Lanka depended on local radicals who claimed links to ISIS.
“Local groups have not been discouraged by ISIS defeats in the Middle East but rather emboldened by them to wage war at home, in accordance with ISIS central directives,” said a report last month by the Institute for Policy Analysis of Conflict on Jamaah Ansharut Daulah and other Indonesian terrorist cells affiliated with the Islamic State.
“The danger has always been that someone with technical expertise and combat experience would return from Syria to turn a handful of ISIS supporters into a more serious threat,” the report added.
Mr. Dedi said that bombs have never been detonated through Wi-Fi networks in Southeast Asia. But such technology has been used in the Middle East, he said.
More than 200 suspected members of Jamaah Ansharut Daulah have been arrested over the last year, along with others from splinter groups and separate cells. The group was formed with the ideological support of Aman Abdurrahman, a radical cleric who pledged allegiance to the Islamic State. A former university lecturer, Mr. Aman is on death row in Indonesia after having been convicted of orchestrating multiple fatal terrorist strikes.
The most populous Muslim-majority nation in the world, Indonesia is a vibrant democracy in which the vast majority of people follow a moderate form of the faith. But Muslim extremists have carried out attacks and called for the nation’s political system, with its guarantee of freedom of religion, to be replaced by a Southeast Asian caliphate that extends across Indonesia, Malaysia and the southern Philippines.
Targets of militant attacks have included hotels and hot spots frequented by foreigners, as well as symbols of the Indonesian state, such as police stations.
The first case of the Islamic State claiming an assault in Southeast Asia occurred in January 2016 when Jamaah Ansharut Daulah militants carried out a bombing and shooting attack in Jakarta’s central business district.
In March, the wife of a suspected Jamaah Ansharut Daulah militant detonated a suicide device during a standoff with the police at her home on the island of Sumatra, killing her child as well. More than 660 pounds of explosives were found at the family’s house, the police said.
Last year, in a chilling heightening of militancy in Indonesia, parents and their children blew themselves up in coordinated attacks on churches and a police station in Surabaya, the second largest city in Indonesia. Those suicide attacks, in which a child as young as 8 was outfitted with explosives, used TATP. The Islamic State claimed responsibility for the attacks.
The deadliest attack in Indonesia’s recent history occurred in 2002 when a local terrorist group linked to Al Qaeda bombed a nightclub on the holiday island of Bali, killing 202 people.
Despite the arrests in recent days, the Indonesian authorities plan to deploy more than 32,000 police and military officers in Jakarta on May 22, when the official election results are tallied, with forces clustered around the election commission headquarters, government buildings and embassies. The Australian Embassy in Jakarta was the target of a suicide car bomb attack in 2004.