The developments illustrate how the Syrian war is sending dangerous ripples across a highly combustible region and sparking fears that jihadis will come home with dangerous ideas and turn their weapons against their own countries.
In Lebanon, where longstanding tensions between Sunnis and Shiites have been heightened by the conflict next door, the fear of blowback has very much turned into reality.
The social fabric of towns and villages across the country is being torn by conflicting loyalties and a wave of bombings carried out by Sunni extremists in retaliation for the Iranian-backed Shiite group Hezbollah's military support of Syrian President Bashar Assad.
In the past few months, at least five Sunni men have disappeared from Bisariyeh, an impoverished, predominantly Shiite village in south Lebanon, and are believed to have gone to fight in Syria.
Two of them -- Nidal Mughayar and Adan al-Mohammad -- returned and blew themselves up outside Iranian targets in Beirut on Feb. 19, killing eight people and wounding more than 100.
"He was a good man with a good heart, but it seems that people who have no conscience brainwashed him," Hisham al-Mughayar said of his 20-year-old son.
As news spread in the village that Nidal was one of the bombers, angry Shiite residents marched to his parents' home and set it on fire along with the family's grocery and four vehicles.
"He destroyed himself and destroyed us with him," said the father, as he took an Associated Press reporter on a tour of his torched, two-story house, much of its furniture reduced to ashes.
Concern about such radicalization has sent Mideast governments scrambling into action.
After years of often turning a blind eye to jihadists taking up arms abroad, Saudi Arabia is enacting new laws and backing a campaign to stop its citizens from joining Syria's civil war.
The intention is to send a clear message that those who defy the law are to fight to the death and are not welcome back.
The move, in part, reflects pressure from Saudi ally the U.S., which wants to see the overthrow of Assad but is alarmed by the rising influence of hard-line foreign jihadists -- many of them linked to al-Qaida -- among the rebels.
Many Saudis have been easy recruitment targets for jihadist organizations. Osama bin Laden and 15 of the 19 hijackers on 9/11 were Saudi.
The oil-rich kingdom was among several nations that backed the anti-communist mujahedeen forces fighting the Soviets in Afghanistan in the 1980s, and Saudi fighters have traveled to other Muslim hotspots around the world since then.
More recently, at the urging of Saudi preachers and even judges, thousands of fighters from Saudi Arabia -- home to a strict, puritanical strain of Sunni Islam -- have joined the 3-year-old uprising against Assad, whose government is dominated by members of his Alawite sect, an offshoot of Shiite Islam.
Saudi officials said fewer than 3,000 Saudis are believed to be fighting in Syria, but analysts and other estimates put the figure as high as 15,000.
While Saudi Arabia continues to support opposition groups in Syria with weapons and other aid, King Abdullah issued a decree in the past month: Any citizen who fights abroad faces three to 20 years in prison.
And anyone who incites people to join foreign wars can get five to 30 years.
"The Saudis are very much concerned about a repeat of the 2004 jihadist insurgency inside the kingdom, led at the time by Osama Bin Laden," said analyst Bilal Saab, referring to a wave of militant attacks inside the country.
"It took time and a considerable amount of resources to counter the insurgency then.
If it were to happen again in today's regional environment where radicalization is on the increase, Saudi counterterrorism efforts will face even more formidable challenges," added Saab, a senior fellow at the Brent Scowcroft Center on International Security at the Atlantic Council.
History is rife with examples of militants returning home from wars with radical intentions.
Thousands of Muslims worldwide who went off to Afghanistan during the 10-year Soviet occupation returned home fired with the fervor of jihad and sought to overthrow their own, sometimes secular-leaning governments.
Many established radical groups in Indonesia, Pakistan, the Philippines, the Caucuses and elsewhere.The shift to criminalize fighting abroad is gaining traction in the Middle East and North Africa. Egypt's military leadership has taken a stricter stand, and Bahrain is drawing up legislation.
Tunisia said it has prevented 8,000 from going to Syria and is putting together a database to monitor hundreds of fighters who have returned.Tens of thousands of foreign fighters have flocked to Syria to take part in the war to topple Assad.
Thousands of Shiites, including Hezbollah fighters from Lebanon, have rushed to Assad's defense.A perpetually troubled country with a weak central government, Lebanon has been a prime victim of the spillover violence.
In Bisariyeh, Hisham al-Mughayar has moved in with his parents. His daughters have stopped going to school, and his other son is no longer going to work for fear of more reprisals.
"Had I known where my son was, I would have gone and got him. We are innocent of all he did," he said. "This is a catastrophe that struck us, although we have nothing to do with it."
By Bassem Mroue and Aya Batrawy