The video of the Indonesian men in Syria emerged shortly before ISIS seized the Iraqi cities of Mosul and Tikrit, in landmark victories on June 10 and 11. It reflects the growing attraction that the Sunni extremist group holds for the most militant jihadists from Indonesia — the country with the world’s biggest Muslim population, and one that has long battled threats of terrorism.

“Like in Syria, the Sunni jihadi movement is split in Indonesia,” Sidney Jones, director of the Jakarta-based Institute for Policy Analysis of Conflict, tells TIME. Some Indonesian jihadists, including many senior leaders of Jemaah Islamiyah (the group behind the Bali bombings in 2002 and other terrorist attacks) are loyal to the alliance around the al-Nusra Front and al-Qaeda, she says, “while most of the more militant, non-JI groups are supporting ISIS.”…

On Saturday, Malaysian media reported that Ahmad Tarmimi Maliki, who bombed an Iraqi military headquarters, earned “the dubious honor of being Malaysia’s first suicide bomber linked to” ISIS. Some months earlier, in November, reports emerged that Riza Fardi, who studied at the infamous Ngruki Islamic boarding school in Central Java — the same school attended by the Bali bombers — became the first Indonesian jihadist to die in Syria.