After the recapture of opposition-held Yabroud by the Syrian regime in March, Abu Mutassem left the rebel ranks and came back home to Lebanon - only to disappear again.
He was among dozens of Lebanese volunteers believed to have fought with the opposition-aligned battalion Jund al-Sham, which disbanded after Syrian military forces regained strategic territory in the Qalamoun region.
The total number of Lebanese rebel fighters in Syria is estimated to be somewhere between 200 and 300, according to the interior minister, the Carnegie Middle East Center, and Tripoli-based sheikhs and militia leaders.
Mutassem's return to Tripoli coincided with a cabinet-backed security plan to quell the intermittent violence that has beleaguered the city since 2008, prompting those with discernible ties to Syrian opposition fighters to flee farther north to Wadi Khaled.
In June, security was tightened to inhibit fallout from the Islamic State's advances in Iraq and to combat armed groups, after the greater Beirut area suffered three suicide bombings in a single week.
"It's as if I'm in a Syrian prison, because I came back here to the security plan," said Mutassem, who spoke to Al Jazeera under a pseudonym while hiding from authorities in Tripoli.
"Our boys who were fighting are either being arrested or are in hiding, so it's as though nothing really changed, as though the Lebanese government is like the Syrian regime."
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The ensuing dragnet led to numerous arrests of would-be "terrorists", which further embittered Lebanon's conservative Salafi community, who perceive the Lebanese army's intelligence wing as a tool of Hezbollah - their rivals both in the political sphere and on the Syrian front lines.
This view has been echoed by religious leaders and residents of majority-Sunni areas in northern Lebanon that were raided.
"It's unjust that everyone who goes to Syria is put in jail while other parties in Lebanon are open about their intervention in Syria," Tripoli-based Sheikh Nabil Rahim, a member of the Committee of Muslim Scholars, told Al Jazeera.
He was referring to an estimated 7,000 Hezbollah members fighting alongside the Syrian regime, a number confirmed to Al Jazeera by Interior Minister Nouhad Machnouk.
With their exposure to extremist elements in Syria, extensive criminal histories, and disenchantment with moderate Sunni leaders, men such as Mutassem are viewed as a significant threat to Lebanon's national security.
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Lebanese officials declined to go into detail, but Mario Abou Zeid, a researcher at the Carnegie Middle East Center who keeps track of their movements, said intelligence reports and inside sources confirm that law enforcement and Hezbollah have been monitoring these men.
Sheikh Masri, a militia leader in Tripoli, opted not to flee in the wake of the army raids. "I'm not afraid of them," he said in an interview from his family home last month. Two days later, he was arrested.
Official government sources, meanwhile, play down the potential threats posed by former members of Jund al-Sham, which is known to have ties to the al-Qaeda-linked rebel group Nusra Front.
"I'm not worried," Machnouk said, crediting the government's security plan. "I think the situation is under control and we took very big steps in drying out the resources of their operations."
The detention of suspected Jund al-Sham members has yielded crucial intelligence that may have helped deter extremist activity inside Lebanon, Abou Zeid said.
"Law enforcement and Hezbollah tried to extract information about rebel groups in Syria from them, and this has helped a lot in preventing terrorism," he said. The majority of members were between the ages of 17 and 29, he said, and were of similar backgrounds.
"Many were unemployed, marginalised in their own community, suffered from injustice, and used the arms trade to generate income. These were all factors that led them to go." Most have not been formally charged, Abou Zeid added.
"I live in fear of checkpoints," Mutassem said. Some of his associates have already been detained, questioned, and released, while others remain in custody without charges. If arrests exceed "acceptable limits", he said, there could be retaliation.
A substantial number of those already implicated in attacks in Lebanon have a history of fighting in Syria.
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Since July 2013, 16 bombings have targeted Hezbollah strongholds and Iranian interests in Lebanon. Five were claimed by the Nusra Front, two by the al-Qaeda-linked Abdullah Azzam Brigades and one, during a security raid in Beirut's Duroy Hotel last month, by the Islamic State group.
Suicide bombers Qotaiba Mohammad al-Satem of Akkar and Mouin Abu Dahr of Sidon both ventured to Syria with the opposition before undertaking their deadly missions.
In the northern town of Fnaydeq, Mahmoud Khaled, who was arrested last month, had also fought among Jund al-Sham's ranks before returning home early this year due to injury. The Lebanese army claims he was planning to use explosives buried on his property to carry out "terrorist activities".
"I suspect they will always be monitored," said Abou Zeid, "because they could easily form a group to destabilise the security situation. These men will always be under a watchful eye to prevent potential threats."
Public sentiment against Syrian President Bashar al-Assad in Tripoli stems from oppression suffered at the hands of the military when Lebanon was under Syrian tutelage. Mobilised under the Tawhid movement, Tripoli's Salafi community was besieged by Syrian troops, and came to a standstill after the 1986 massacre in Bab al-Tabbaneh.
...they could easily form a group to destabilise the security situation. These men will always be under a watchful eye to prevent potential threats.
On the front lines in Syria, Jund al-Sham distinguished itself as one of the most active battalions in the opposition. By the time the group disbanded, many members had gravitated towards other more extremist elements.
Battalion leader Khaled Mahmoud al-Dandashi, known as Abu Suleiman, died amid regime strikes, holed up in Qalaat al-Hosn. An earlier video circulated by Jund al-Sham stated that the group sought to carry out "jihad to enable God's rule on earth".
Although the 2011 uprising was a catalyst, the conditions for recruitment in Lebanon were ripe long before the Syria crisis. With more than twice the national average of extreme poverty and the lowest levels of per capita government expenditure, residents of northern Lebanon have long felt disadvantaged.
The failure of moderate Sunni leaders to address their interests, especially after Syria's withdrawal from Lebanon in 2005, left some disillusioned and prompted them to turn to local militias.
Efforts to improve the situation have been mired in politics, Tripoli-based anthropologist Maha Kayal told Al Jazeera.
Kayal was part of a team that devised an economic strategy for the region, the centrepiece of which was to establish a canning factory in the mainly Sunni Bab al-Tabbaneh neighbourhood, where the average adolescent boy quits school and begins working at the age of 14.
The strategy died after a parliamentary committee review, with politicians citing financial costs.
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For Mutassem, when he was not in Syria, he was helping to shuttle people, arms, and money from Lebanon to and from the Syrian city of Talkalakh.
In the beginning they carried light weapons, such as AK-47s and RPGs, but by the end they were using B-10 recoilless rifles, a testament to the enterprising Sheikh Masri, who was influential in acquiring weapons.
The smuggling came to a halt, however, when the sheikh could not provide the battalion with requested anti-aircraft missiles.
Hunched with folded arms, Mutassem, once a chauffeur, said he still dreams of days spent in battle: "There's nothing like it."