JAKARTA, Indonesia — An audacious attack in the heart of Indonesia’s capital by suicide bombers was funded by the Islamic State group, police said Friday, as they arrested three men on suspicion of links to the plot and seized an IS flag from one of the bombers.

National police chief Gen. Badrodin Haiti told reporters that Thursday’s attack was funded by IS through Bahrun Naim, an Indonesian who spent one year in jail for illegal possession of weapons in 2011, and is now in Syria fighting for the Islamic State.

Supporters of the Islamic State group also circulated a claim of responsibility for the attack on Twitter late Thursday. The radical group controls territory in Syria and Iraq, and its ambition to create an Islamic caliphate has attracted some 30,000 foreign fighters from around the world, including a few hundred Indonesians and Malaysians.

The IS link, if proved, poses a grave challenge to Indonesian security forces. Until now, the group was known only to have sympathizers with no active cells capable of planning and carrying out a plan such as Thursday’s in which five men attacked a Starbucks cafe and a traffic police booth with hand-made bombs, guns and suicide belts. They killed two people — a Canadian and an Indonesian — and injured 20. The attackers were killed subsequently, either by their suicide vests or by police.