After four months allegedly spent fighting in Syria alongside an al-Qaeda affiliate, an Arab citizen of Israel was covertly arrested upon his return to Israel in April and indicted on Wednesday.

“The trend of Arab Israelis leaving for Syria entails great danger to the State of Israel,” the Shin Bet internal security agency said in a statement, noting that the arrested man, Ahmed Imad Shurbji, a resident of the northern town of Umm al-Fahm, underwent combat training in Syria, participated in battles, and was indoctrinated in jihadist ideology.

The trickle of Arabs from Israel to fight on the Sunni side of the civil war in Syria is of serious concern to the Shin Bet. In January the organization arrested three men, including Ayad Abu-Sara, an East Jerusalem resident who was allegedly set to leave for Syria — part of a plan that was to culminate in suicide attacks against the US Embassy in Tel Aviv and Jerusalem’s International Convention Center.

There are currently, according to a study by the Meir Amit Intelligence and Terror Information Center, over 5,000 foreign Arab fighters in Syria, constituting “the spine” of the fighting ranks of the jihadist groups Jabhat al-Nusra and Islamic State of Iraq and the Levant.

The study, released this week, pointed at Saudi Arabia, Jordan, Egypt, Tunisia, and Libya as contributing the bulk of the warriors, with only 30 from Gaza and “several individuals” from the Palestinian Authority and Israel.

Shurbji, 23, left Israel on January 16, according to the indictment, despite warnings from Israeli security services. Three friends — Rabia Shehadeh, Ahmed Habshi, and Muhammad Jabbarin — left with him but each traveled a different route so as to avoid suspicion. The other three men have yet to return to Israel.

The four of them met in Turkey and were escorted into Syria, where they joined a fighting group called Jesh Muhammad. After brief paramilitary training, Shurbji joined the Islamic State of Iraq and the Levant, one of the two dominant al-Qaeda affiliates fighting against President Bashar Assad’s regime in Syria.

Shurbji was given basic guerrilla warfare training, including the handling of a rifle, a PKC machine gun, and RPG anti-tank rockets. He confessed to having taken part in several battles, firing his AK-47 rifle and manning checkpoints in Syria.

Shurbji returned to Israel on April 20 and was arrested at Ben-Gurion International Airport. He has been charged with participation in unlawful military activities and illegal departure from the country.