Counter-terrorism investigators are working to establish how the two suicide bombers behind the attack in East Jakarta on Wednesday night are linked to the Islamic State in Iraq and Syria (ISIS) group.
The duo were identified as Solihin, an administrative employee at Darul Anshor, an Islamic boarding school in Poso, Central Sulawesi; and Ichwan Nurul Salam, 34, from Bandung, West Java.
The attack near a bus terminal in Kampung Melayu also killed three policemen and injured 11 others, including a 17-year-old driver and 19-year-old student.
A police source told The Straits Times yesterday that while one of the bombers had ties to militants in Poso, both may have ISIS links.
This is because their use of low- grade explosives and aluminium shrapnel in the "pressure-cooker bombs" on Wednesday was similar to the modus operandi of Indonesian militants loyal to ISIS who targeted police in previous attacks.
On Friday (May 26) morning, the militant group claimed responsibility for the attack. "The executor of the attack on the Indonesian police gathering in Jakarta was an Islamic State fighter," the group's news agency Amaq said. No more details were available.
Solihin and Ichwan struck at two spots about 10m apart near a Transjakarta bus shelter. The first bomb went off outside a public toilet at about 9pm local time and the second just minutes later near a bus- stop shelter, police spokesman Setyo Wasisto said yesterday.
Preliminary investigations indicated that the duo's target was a group of police officers who were in the area to escort a parade ushering in the Muslim fasting month of Ramadan, which starts tomorrow.
Singapore leaders condemn bombing
President Tony Tan Keng Yam and Prime Minister Lee Hsien Loong have condemned the bombing in Jakarta, and expressed their sadness on behalf of Singapore in separate letters to Indonesian President Joko Widodo.
Dr Tan wrote that he was "deeply saddened" by the blast.
"Our thoughts and prayers are with the loved ones of those affected," he wrote.
He extended Singapore's "deepest condolences to the families of the victims".
"Singapore strongly condemns this violent and callous act of terror which has caused the loss of innocent lives and many injuries," he wrote.
"We stand in solidarity with the people of Indonesia during this difficult time."
In his letter, PM Lee wrote that he was "shocked and saddened to hear of the bomb blast" at the Kampung Melayu Bus Terminal on Wednesday.
"The loss of lives and multiple injuries caused to the police and civilians were tragic, especially coming right before the holy month of Ramadan," he wrote.
"On behalf of the Government of Singapore, I offer our deepest condolences to the bereaved families. We condemn this senseless act of violence.
"We stand together with Indonesia against terrorism. I am confident that the Indonesian Government and people will overcome this swiftly."
Police found a sales invoice dated May 22 from a store in Padalarang, West Java, showing the purchase of a pressure cooker, metal plates, nails, ball bearings, cable switchers and other bomb-making materials.
Inspector-General Setyo said the attack is similar to one in Bandung, West Java, in February by a lone militant suspected of having links to a radical network sympathetic to ISIS.
Indonesian President Joko Widodo condemned the attack, which came just days after a suicide bombing in the British city of Manchester killed 22 people, including children and teenagers. "This is execrable. Ojek (motorcycle taxi) driver fell victim, public mini-van driver, store sellers, as well as policemen," he told reporters in his home town in Central Java, referring to the three dead police officers and 11 people injured.
Insp-Gen Setyo told TVOne news station that local cells might have been spurred to act after the suicide bombing in Manchester and an attack by Islamist militants in Marawi in the southern Philippines.
Indonesia's state-owned airport operator Angkasa Pura II tightened security at Soekarno-Hatta International Airport in Jakarta yesterday.
"The security in and outside of the airport has been intensified," the airport's security rescue and fire senior manager Tommy Bawono told Tempo news.
Wednesday's attack was reminiscent of a suicide bombing in Solo on July 5 last year. Just days before the fasting month was to end, a suicide bomber riding a motorcycle blew himself up after he was stopped outside the local police headquarters. He also used low-grade explosives in the homemade bomb, which contained ball bearings and screws.
Indonesia, which has the world's largest Muslim population, has long struggled with Islamic militancy. Hundreds of radicals have left the country for the Middle East to fight alongside ISIS militants.