A prominent Lebanese activist known for his opposition to the Shiite movement Hezbollah was found dead in his car in the country's south on Thursday, a senior security official said.

Lokman Slim had not been heard from by his family since getting in his car on Wednesday evening to drive back to Beirut.

"He was found dead in his car, killed by a bullet to the head," the official said on condition of anonymity. Lebanese media also reported his death.

A journalist, political analyst and activist known as one of the leading Shiite voices criticising Hezbollah, Slim was regularly attacked in media loyal to the powerful group.

Security sources did not immediately elaborate on the circumstances of his death but Slim's own sister said before his death was even confirmed that his disappearance was inevitably linked to his opinions.

"He had a political stance, why else would he have been kidnapped," Rasha al-Ameer told AFP.

Slim was often criticised by Hezbollah supporters for being instrumentalised by the United States.

The secular intellectual and pro-democracy activist had also made several documentary films with his wife Monika Borgmann, who had sounded the alarm on social media when her husband went missing.

Lokman Slim: daring Lebanese activist, admired intellectual

Lebanese intellectual Lokman Slim, found shot dead Thursday at age 58, was an outspoken critic of the Shiite movement Hezbollah and an advocate for preserving the memory of his country's civil war.

The son of a prominent lawyer and an Egyptian mother, Slim was an activist, writer, publisher and filmmaker, as well as a leading secular voice in the Shiite Muslim community.

He advocated curbing the influence of Hezbollah, the pro-Iran and anti-Israel political party and armed group that has millions of followers in Lebanon but is labelled a terrorist group by the US, EU and other governments.

Slim also spent years working relentlessly to preserve Lebanon's collective memory of its 1975-1990 civil war and carried out acclaimed research on topics such as mass graves and transitional justice.

He created an organisation called Umam Documentation and Research with the aim of building an unparallelled archive of the civil war, arguing that Lebanon could not move forward until it had dealt with its past.

He was known for his love of books and mastery of the Arabic language.

His home in the southern Beirut suburbs, known as Villa Slim, was a hub of cultural activity, where several languages could always be heard and where film screenings and exhibitions were often organised.

Despite having been singled out by Hezbollah supporters as an enemy and frequently accused of being on the US embassy's payroll, he used to tell his friends he was not worried about his safety.

He would argue that should he be killed, everybody would know who was behind it. 

"He used to tell me he was not afraid of death," his sister Rasha al-Ameer told AFP. 

"They have killed an exceptional human being."

House always open

His friend Hana Jaber said her brother taught Slim in high school, and was impressed by his pupil's critical mind.

The history teacher "was always coming home with a story about him, an anecdote, a comeback", she said.

As an adult, Jaber got to know a man she described as having great "nobility of spirit".

"He was an elegant person of steady convictions and well-established qualities. He had nothing to prove. He advanced, modestly and with confidence," the researcher in modern history said.

While Slim's murder put the spotlight on his political activism, his greatest legacy will likely be his relentless commitment to fostering a collective Lebanese memory and spirit of accountability.

"Lokman Slim was at the forefront of the struggle against impunity in post-war Lebanon," said Amnesty International's regional director Lynn Maalouf.

In 2008, he founded the Haya Bina association with the goal to "defend principles of citizenship, tolerance, pluralism, democracy and human rights".

During cross-sectarian pro-democracy protests in late 2019, thugs loyal to Hezbollah and the other main Shiite party Amal were involved in several violent incidents.

They beat up protesters and in one instance in December that year plastered messages on the walls of Slim's home calling him a traitor and warning that his "time will come".

But he lived with no personal security and the gates to his house were famously always open.

Publishing house, documentaries

After studying philosophy in Paris, Slim founded the Dar al-Jadeed publishing house, which promoted new and controversial writers and, for the first time, translated works by former Iranian president Mohammad Khatami into Arabic.

"As soon as he started Dar al-Jadeed, he made very original choices," said Jaber.

Slim was an essayist himself and also made several documentary films with his German-Lebanese wife Monika Borgmann.

Their film "Massaker" -- which studied six perpetrators of the 1982 Christian militia massacres of 1,000 people at the Sabra and Shatila Palestinian camps in Beirut -- was awarded the Fipresci Prize at the 2005 Berlinale.

It was during the production of "Massaker" that he and Borgmann discovered that Lebanon had neither a central archive nor a national library, leading them to found Umam in 2005.

In their 2016 film "Tadmur", Lebanese men recounted their ordeal as detainees in Syria's infamously brutal Palmyra prison.

He had recently been working on a project to document Syria's almost 10-year-old civil war.