SURABAYA, Indonesia—A militant attack on a police post extended a deadly wave of Islamic State-linked terrorist strikes in Indonesia on the eve of the Muslim holy month of Ramadan, after police uncovered a cache of pipe bombs and bomb-making equipment in the country’s second-largest city.
Five assaults on churches and police buildings in the past four days have killed at least 14 security personnel and civilians, making them together Indonesia’s deadliest since 2005. At least 18 assailants have been killed in the attacks and ensuing police operations, as well as in what appeared to be an additional failed plot when a bomb exploded in an apartment Sunday night.
Police have tied the onslaught to a terrorist cell that two years ago was responsible for the first Islamic State-linked attack in Indonesia, the world’s most populous Muslim-majority nation. National police chief Tito Karnavian told reporters earlier this week that investigators suspect the attacks were motivated by instructions from Islamic State’s central command.
The terror blitz is the result of pro-Islamic State extremists wanting to “show their existence and their ability to attack,” said Muh Taufiqurrohman, a Jakarta-based analyst at the Center for Radicalism and De-radicalization Studies. “Unless police apprehend pro-ISIS extremists they are tailing, we should expect more attacks to come.”
In the latest violence on Wednesday in Riau province, on the island of Sumatra, police said they shot dead four men who attacked a police headquarters, two of them brandishing swords. One officer was killed.
Police suspected one of the attackers had a bomb strapped to his chest, according to an internal report reviewed by The Wall Street Journal. The report said at least one of the attackers killed by police was believed to be a member of a pro-Islamic State terrorist cell that investigators say is responsible for the other attacks in recent days, on the main island of Java.
Wednesday’s assault, for which there was no immediate claim of responsibility, follows suicide bombings by separate families, including children and women, on Sunday and Monday on churches and a police headquarters in the city of Surabaya. Islamic State claimed responsibility for those attacks, which it called a “martyrdom-seeking operation.”
In the wake of the Surabaya attacks, police said they found pipe bombs and bomb-making materials at the home of one of the bombers in a middle-class suburb of the city, and that they had disposed of dozens of bombs seized in various raids since the attacks began. On Tuesday, investigators cordoned off a block of a street around the home as hundreds of onlookers gathered while police worked to deactivate explosives. Police bomb-disposal officers detonated at least one device on the premises, sending a deep thud across the neighborhood.
Indonesia has long faced a terrorist threat from radical Islamists. But the country has had a high degree of success in hampering the capabilities of extremist groups who have waged occasional attacks across the archipelago.
In the country’s deadliest attack, 202 people, mainly Western tourists, were killed on the resort island of Bali in nightclub bombings in 2002. Targets since then have included upscale hotels in Jakarta and the Australian embassy. More recent attacks, including those linked to Islamic State, have been mostly aimed at police and security officials.
—Anita Rachman and Joko Hariyanto contributed to this article.