Bambang Sukirno, another Ngruki graduate and a Bashir associate, took part in a humanitarian mission to Latakia last year, according to video interviews he gave to Islamist media on his return. Sukirno published the autobiography of Bali nightclub bomber Imam Samudra, who writes lovingly of his experience fighting jihad in Afghanistan.
"We have learned that some of our alumni are involved in the struggle in Syria, but once again I reiterate that we can't monitor or follow what our students do after they graduate," said Wahyudin, Ngruki's principal. The cleric, who goes by a single name, used a similar defense when confronted with the fact that former students and teachers were convicted of carrying and planning out terrorist attacks inside Indonesia in the 2000s.
Ihsani's father, Sholeh Ibrahim, has been a teacher at the school for years, and heads the extremist Jamaah Ansharut Tauhid organization in Solo, where the school is located. JAT is campaigning for Islamic law in Indonesia, is anti-Christian and supports al-Qaida's vision. At least 30 members have been convicted for terrorist offenses over the last four years, and the U.S. State Department declared it a foreign terrorist organization in 2012.
The head of the organization nationwide, Abu Bakar Bashir, is serving a 15-year jail sentence for supporting the establishment of a militant training camp. From behind bars, the cleric issued a call for jihad to Syria year.
Ibrahim said he last spoke to his son Aug. 21. He didn't mention any travel plans, but asked about his family in Indonesia and spoke of his activities at college in Islamabad, Pakistan, a popular destination for Indonesians looking for cheap degrees in Islam. Ibrahim said neither he nor any of his son's friends have heard from him since.
Despite being a proponent of jihad, Ibrahim said he was worried.
"Honestly speaking, as father, I'm concerned," said Ibrahim. "But I trust in Allah and his will, and I'm sure he (Ihsani) will choose a blessed path."
A sustained crackdown by Indonesian authorities since 2002 has reduced the threat of large-scale terrorism against Western or civilian targets in Indonesia and elsewhere in the region. But small groups of militants continue to plot, train for and carry out attacks, mostly against police targets, across the country of 240 million people.
Syria represents a rare training and battle opportunity for the current generation of Indonesian militants.
Most of the foreign fighters in the country come from the Middle East. Estimates of the numbers of Western European fighters range from 396 to 1,937, according to a recent study by the International Centre for the Study of Radicalization.
It's unclear where or with whom the Indonesians are fighting. According to the center, most of the foreigners are grouped with the Nusra Front or the Islamic State in Iraq, the two opposition brigades that are closest to al-Qaida.
"Anybody coming back from Syria is going to have immediate credibility and legitimacy in the jihadi movement," said Sidney Jones, the director of the Institute for Policy Analysis of Conflict. "There might be people coming back who can take any of these amorphous, feckless groups of extremists and drill them into shape."
While the country's extremist fringe is rallying around Syria, it is also apparent most mainstream Indonesian Muslims are not signing up to the cause because it means having to embrace the uncompromising — and still unpopular — sectarian vision that is at the heart of the conflict.
Only around 20 people showed up at a recent meeting at a mosque in west Jakarta organized by hardliners who had returned from a Syrian humanitarian mission. A question from a reporter as to why Indonesians should take sides in a civil war in a Muslim country when other causes, for example Palestine, were still pressing, was met with a smattering of applause from those present.
Joserizal Jurnalis, a doctor who has led humanitarian missions to help Muslims in Afghanistan, Lebanon and elsewhere, has angered many fellow Indonesian Islamists by refusing to go to Syria or supporting the cause.
He says those rallying around Syria are "those close to al-Qaida only."
"It's a sectarian war. It's not clear to me why we should be helping in the slaughter of other Muslims," he said.
Follow Chris Brummitt on Twitter at @cjbrummitt
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