The ease with which three separate groups of ethnic Uighur militants sneaked into Indonesia is ringing alarm bells for security forces, who are on high alert for a far deadlier attack than last week's assault on Jakarta.
At least 10 Uighurs, who hail from China, arrived in the world's most populous Muslim nation over the last 18 months to join Islamist radicals, exposing an extensive support network ready to welcome wannabe jihadis.
Police fear the same network could assist in the return of battle-hardened Indonesian Islamic State fighters from Syria, who could then launch more calculated attacks, similar to that which hit Paris last November.
Indonesia's sprawling archipelago of 18,000 islands is too vast to control the movements of militants, drug smugglers, human traffickers and refugees, police say.
"There is enough security at the main entry points," the country's police chief, Badrodin Haiti, told Reuters. "But there are more traditional points for entering illegally, where usually fishermen bring people in."
With around 500 Indonesians taking one-way trips to join Islamic State in Syria and Iraq, authorities had not considered the Southeast Asian nation's porous border to be a pressing security issue until recently.
But last month's arrest of an Uighur individual, Ali Mehmet, after police found bomb-making equipment in a house in a Jakarta suburb, spotlighted how easy it is for people to be smuggled into Indonesia.
Uighurs come from Xinjiang in far western China, a region Beijing says is home to Islamist militants and separatists.
Jakarta says it is working with China to stem the flow of Uighur militants, who police say are responding to a call by Santoso, Indonesia's most high-profile backer of Islamic State, to join his band of fighters.
Fishermen from the Indonesian island of Batam, an area notorious for trafficking, were suspected of helping Mehmet to cross over from neighbouring Malaysia, his lawyer, Asludin Hatjani, told Reuters.
"Clearly they communicate with people here before arriving, mainly via Whatsapp," he said, referring to a social media application widespread on mobile telephones.
Using the underground support network, the former bread vendor made his way to the Jakarta suburb where, authorities believe, he and his housemates plotted an attack on the capital.
"It is clear from the police evidence that Mehmet was caught living in a house where there were bomb-making materials and he was with people related to the plan," Hatjani said.
Mehmet was innocent, however, he said, adding that his client had been at the wrong place at the wrong time. "As his lawyer, I can say that he was here for tourism purposes and stress relief after getting divorced from his wife."
But police suspect Mehmet was one of several militants, including the four Indonesians who launched last week's Jakarta attack, who received funding from Islamic State members in Syria.
Two more Uighurs were arrested this month after police found bomb-making equipment at their house.
Police say they are starting to identify network members who are in jail or on the run. But with at least 1,000 supporters of Islamic State in Indonesia, experts say fighters returning from Syria could receive ample help.
"The network of Islamic State sympathizers is quite widespread and they are believed to be in several locations," said Jakarta-based terrorism expert Rakyan Adibrata, who advises parliament.
Authorities want to prevent a rerun of history.
In the 1980s and early 1990s, Indonesians and Malaysians returning from battling the Soviet Union in Afghanistan formed the transnational group Jemaah Islamiyah, which was responsible for the 2002 Bali bombings that killed more than 200 people.
In response to the Jakarta assault, which killed eight people, including the four attackers, Indonesian President Joko Widodo agreed to new rules allowing security forces to arrest returning Islamic State fighters.
But spotting them on their return home remains the problem.
"In any given place, there can be hundreds of points of entry," Haiti said.