Hoda al-Ali searched for her son for 34 years, until the day she drew her final breath.

In 1986, when he was 18, he was arrested at a checkpoint by Syrian troops occupying Lebanon. A single mother of 10, Ms. al-Ali would scrape together what little she earned as a seamstress and travel every few months into neighboring Syria to scour its prisons. When her legs grew frail, her children took over, chasing the same breadcrumbs, never finding answers.

Then, in December, a video emerged.

President Bashar al-Assad of Syria had just been toppled by a lightning rebel offensive. In the chaos, a news crew filming outside a Syrian prison captured the image of an older man, disheveled and dazed, emerging from its gates. The family froze. They were sure it was the missing son, Ali, and their story quickly made headlines.

But days passed. Then weeks. Ali never returned. Hope faded. Lebanese officials offered no answers. Journalists stopped calling.

Months later, the search grinds on.

“We need to continue my mother’s mission,” said Ali’s brother Moammar, clutching an old photograph of him in the family’s home in northern Lebanon. “We still have hope he is alive.”

After the collapse of the Assad government, prison doors across the country flew open, and Syrians flooded in to search for traces of their friends and loved ones who had disappeared in untold numbers under the brutal regime. In Lebanon, however, many could only watch and wait.

Thousands of Lebanese had gone missing during Syria’s decades-long occupation of their country, which lasted from 1976 to 2005, and many were believed to be imprisoned in Syria. For years, the tentacles of Mr. al-Assad’s security state extended well beyond Syria’s borders, ensnaring not only political opponents, but also ordinary civilians caught up in its machinery of suspicion. Dissidents, laborers, businessmen — anyone could vanish.

The disappearances became a hallmark of Syria’s rule, sometimes aided by pro-Syrian Lebanese factions, with men and women taken from their homes or snatched off streets. Behind the checkpoints, Damascus’ secret police, known as the mukhabarat, ran detention sites across Lebanon — Beirut’s luxury Beau Rivage Hotel became shorthand for torture — and routinely transported suspects across the border to prisons like Sednaya.

When Mr. al-Assad was toppled, Lebanese officials estimated that more than 700 of their citizens were still imprisoned in Syria, but some advocacy groups say there are far more unaccounted for. So far, only nine have returned, many after languishing in prison for decades.

Among them was Suheil Hamawi, who was taken from his home more than three decades ago.

Standing on his balcony overlooking the Mediterranean, Mr. Hamawi, 61, took a drag from his cigarette and gazed at the crystal clear waters below. It was a lot to take in: He had not seen the sea in 33 years.

“These scenes — you can only repeat them in your mind, and in your dreams,” Mr. Hamawi said. “I feel like I can breathe again.”

A member of a Lebanese Christian political party opposed to the Syrian occupation, Mr. Hamawi was abducted from his family home in 1992 by Syrian intelligence officers and taken across the border.

His capture was so abrupt that for the first 17 years, his wife believed he had simply vanished. His son, Georges, was only 10 months old at the time. When Mr. Hamawi returned home, Georges was 33 and had a son of his own.

Mr. Hamawi spent the days and weeks after his release sipping cups of cardamom-infused coffee and video-calling relatives, many of their faces now unrecognizable to him.

“Do you remember my children?” said a cousin over the phone, sitting alongside her adult daughter.

“When I left her, she was so small,” Mr. Hamawi said in disbelief.

Former Lebanese prisoners interviewed by The New York Times described brutal treatment and torture, which they said was often more severe on account of their nationality.

Families of those missing said they had received no help from the Lebanese authorities while Mr. al-Assad was in power, and had often been forced to spend thousands of dollars on bribes to Syrian security forces to get a sign of life from their relatives or win their release.

Bounced around the Assad regime’s network of prisons, Mr. Hamawi was first detained in Palestine Branch in Damascus, and later in the notorious Sednaya. He spent the first five years in solitary confinement in a cell about 30 inches wide and six and a half feet high. He described it as “a tomb with a door.”

“There was no light at all,” he said. “We used to recognize day from night by the sound of birds, or from the type of food they used to give us.”

In Sednaya, Mr. Hamawi grew close with his cellmates. They had been stripped of their names and given numbers — his was 55 — but that did not stop them from forging quiet bonds.

Most did not live to see the fall of the Assad regime, he said.

One Syrian friend, a journalist, disappeared after being told by guards that he had a visitor, only for Mr. Hamawi to find out 10 years later that he had been executed. Another close friend, Fahed, a fellow Lebanese, refused treatment after becoming seriously ill, preferring to die than endure another day.

“He was stronger than me,” Mr. Hamawi said. “He accepted death, and I couldn’t.”

In recent months, Syria’s new rulers established a commission to investigate the fates of those who disappeared as part of a broader push for transitional justice.

In Lebanon, where Syria’s shadow still looms, families of the missing have been fighting a parallel battle for decades, pressing for accountability and answers.

But the Assad regime wielded outsize influence over Lebanon, refusing to shed light on the fate of the vanished, and Lebanese officials were often unable — or unwilling — to press the issue.

For Abir Abou Zeki, whose father was among the disappeared, it has been a lifelong fight.

It was June 12, 1987, when her father, Khalil, walked into her bedroom in a home just south of Beirut and planted a soft kiss on her cheek for the last time.

The family of five was set to begin new lives in Germany. Their passports were ready. So were their plane tickets. But Khalil had to make one last business trip into Syria to collect spare truck parts for the company he worked for.

When days went by and he did not return in time for their flight, panic set in. They eventually learned he had been arrested because he had a coffee tin containing American dollars, a criminal offense under the Assad regime.

Then he disappeared.

In the months that followed, their family fell apart. Ms. Abou Zeki’s mother, Dalal, walked out, unable to cope with the stress of raising three children alone.

Their dreams of a new life had been shattered almost overnight.

For years, members of Lebanon’s Druze community, a close-knit religious minority to which the family belongs, rallied around the family. Relatives and local officials made repeated appeals to the Syrian authorities.

“The only answer was: ‘Yes, he is in our prisons. Consider him ours and don’t ask about him anymore,’” Ms. Abou Zeki said.

Years later, the message hardened: “Consider him dead,” she recalled.

When Mr. al-Assad was toppled in December, Ms. Abou Zeki, like so many others, allowed herself a flicker of hope — though it came tangled with guilt and fear.

“I think it’s selfish of me to say that I want him to be alive after all that torture,” she said. “But we suffered a lot. We used to feel guilty if we were eating or drinking — because he wasn’t able to. We felt warm while he was cold.”

“I think it’s easier to know that he died,” she said quietly.

Others are hoping wholeheartedly.

In the Ali family’s crumbling home in northern Lebanon, Moammar clutched the portrait of his brother Ali, taken when he was still a young man. As their mother once did, he was saving up for a trip across the border, hoping to find answers.

From the window, Syria was visible in the distance. Somewhere out there, they still believed, Ali is alive, waiting to be found.

“A mother’s feeling is never wrong,” Moammar said.

Dayana Iwaza contributed reporting.