JAKARTA, Indonesia » Indonesian police today arrested three men on suspicion of links to the brazen attacks in the heart of the country’s capital, and said they recovered a flag of the Islamic State group from the home of one of the attackers.

The discovery of the flag bolsters authorities’ claim that the attack Thursday was carried out by the Islamic State group, which controls territory in Syria and Iraq and whose ambition to create an Islamic caliphate has attracted 30,000 foreign fighters from around the world, including a few hundred Indonesians and Malaysians.

The arrests of the three took place at dawn at their homes in Depok on the outskirts of Jakarta, police said in a text message, citing Col. Khrisna Murti, director of criminal investigations who led the raid. It said they were arrested for suspected links to the attackers. MetroTV broadcast footage of the handcuffed men being escorted by police.

Five men attacked a Starbucks cafe and a traffic police booth with hand-made bombs, guns and suicide belts Thursday, killing two people — a Canadian and an Indonesian — and injuring 20. The attackers were killed subsequently, either by their suicide vests or by police.

National police spokesman Maj. Gen. Anton Charliyan told reporters a black IS flag was found in the home of one of the attackers and police believe they have established their identities.

He says two of the five men were previously convicted and imprisoned for terrorism offenses.

The IS link, if proved, poses a grave challenge to Indonesian security forces because until now the group was known only to have sympathizers with no active cells capable of planning and carrying out such an attack.

In recent years Indonesian anti-terror forces had successfully stamped out another extremist group known as Jemaah Islamiyah. It was responsible for several attacks in Indonesia, including the 2002 bombings of bars in Bali, which left 202 people dead, as well as two hotel bombings in Jakarta in 2009 that killed seven people.

Terrorism experts say IS supporters in Indonesia are drawn from the remnants of Jemaah Islamiyah.

A few hundred Indonesians are known to have traveled to Syria to join the IS. Few have come back. Still, police believe that an Indonesian IS fighter, Bahrum Naim, who is in Syria may have inspired and instigated the Jakartaattack.

Jakarta residents were shaken by Thursday’s events but refused to be cowed.

The area near the Starbucks cafe remained cordoned off with a highly visible police presence. Onlookers and journalists lingered, with some people leaving flowers and messages of support.

A large screen atop the building that houses the Starbucks displayed messages that said “#prayforjakarta” and “Indonesia Unite.”

Newspapers carried bold front-page headlines declaring the country was united in condemnation of the attack, which was the first in Indonesia, the world’s largest Muslim-majority nation, since the hotel bombings in 2009.

Risti Amelia, an accountant at a company near the Starbucks said she was “still shaking and weak” when she returned to her office Friday. But because staff remained emotional, the company decided to send workers home, she said.

Supporters of the Islamic State group circulated a claim of responsibility for the attack on Twitter late Thursday.

The message said attackers carried out the Jakarta assault and had planted several bombs with timers. It differed from Indonesian police on the number of attackers, saying there were four.

The statement could not be independently verified by The Associated Press, though it resembled previous claims made by the group.

Taufik Andri, a terrorist analyst, said although the attack ended swiftly and badly for the attackers, their aim was to show their presence and ability.

“Their main aim was just to give impression that ISIS’ supporters here are able to do what was done in Paris. It was just a Paris-inspired attack without being well prepared,” he told The Associated Press. Those attacks in November killed 130 people.