Some 40 Palestinians are being held in Palestinian Authority prisons on suspicion of having fought alongside Syrian rebels against the forces of President Bashar Assad, senior Palestinian sources told The Times of Israel on Thursday. The fighters were arrested upon their return to the West Bank.

During questioning the prisoners revealed that they had joined a variety of radical Islamic organizations that are active in the civil war in Syria, including Jabhat al-Nusra and the Islamic State of Iraq and the Levant, both of which are affiliated with al-Qaeda and designated as terror organizations by Israel and the United States, the sources said.

They noted that Palestinian security forces had also recently arrested nearly 30 people suspected of being members of a radical Salafist jihadi network in the West Bank.

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Israel’s Shin Bet security service has warned that the civil war in Syria, a magnet for aspiring jihadists throughout the Middle East, has deepened the roots of al-Qaeda and like-minded organizations in the region. Those groups, it said, were increasingly striving to link up with willing Palestinians in the West Bank and East Jerusalem in order to strike Israel and Western targets.

In January, two East Jerusalem residents were indicted at the city’s District Court for allegedly hatching a plot with al-Qaeda to carry out numerous terror attacks including suicide bombings at the US Embassy in Tel Aviv and Jerusalem’s International Convention Center, as well as the abduction of an IDF soldier.

Last November, a special Israeli anti-terror force killed three members of a Salafist group in a shootout in the West Bank city of Hebron. Israeli defense officials say there is some cooperation with their Palestinian counterparts in the West Bank to keep the Salafis under watch.

The Palestinian sources who spoke to The Times of Israel on Thursday claimed that the PA was determined to disrupt the efforts of radical groups to make inroads in the West Bank and sow widespread violence and terror. In addition, they noted, the PA was acting to foil attacks initiated by Hamas and Islamic Jihad and has recently had some success to that end.

Still, the PA security apparatus has scaled back its activities inside the Palestinian refugee camps as more and more residents of those overcrowded and poverty-stricken camps have become radicalized and increasingly disenchanted with Fatah’s rule.

In December a report by the Meir Amit Intelligence and Information Center in Herzliya found that a growing number of Palestinians from the Gaza Strip were making their way to Syria to fight alongside rebel groups. Seven Gaza natives, among them former members of the ruling Hamas organization, have so far been killed in action, including three as a result of suicide attacks, the report said.

In addition, between 10 and 15 Israeli Arabs have traveled to Syria to take part in the fighting there, the report said.

Palestinian and Israeli Arab fighters in Syria mostly enter through Turkey, the report found, some of them after a trip to Saudi Arabia as pilgrims on Hajj, the annual Muslim mission to Mecca.

Despite their relatively low absolute numbers, the report raised concerns that the Palestinians may become radicalized on the battlefield and subsequently carry out attacks against their own governments — whether in Gaza or Israel — upon their return home. Those concerns, it appears, must now be extended to include the West Bank.

Elhanan Miller and AP contributed to this report.