Saif al-Islam Gaddafi, the son of the late Libyan dictator Muammar Gaddafi and for years the second most powerful person in the country, has been killed in a village south-west of Tripoli, officials said on Tuesday night.

The 53-year-old died from gunshot wounds in the town of Zintan, 85 miles south-west of the capital, according to the Libyan attorney general’s office. Gaddafi’s own office said he was killed in his home by masked assailants.

Once seen as a pro-western reformer who might usher Libya towards constitutional change, Gaddafi quickly backed his father’s violent crackdown on nationwide popular protests in 2011. The international criminal court in the same year issued a warrant against him for crimes against humanity over the repression, an accusation echoed by a Tripoli court in 2015.

The Libyan chief prosecutor’s office said it was looking for suspects and had dispatched forensic experts to the village, but did not provide further details of the killing.

According to Gaddafi’s office, four masked men had stormed his house, turned off its cameras and clashed with him before killing him, in what it described as a “cowardly and treacherous assassination”. His sister, by contrast, told Libyan TV that he had died near the border with Algeria.

Khaled al-Mishri, the former head of the Tripoli-based High State Council, demanded an “urgent and transparent investigation” into the killing.

Despite not having a formal role, Gaddafi was one of the most important symbolic figures of the post-2011 era in Libyan politics. Analysts warned his death could inflame pro-Gaddafi factions in the country.

Gaddafi studied at the London School of Economics and was groomed to take over from his father. He was styled as the pro-western, pro-reform face of Libya, and led delegations aimed at ridding Libya of its weapons of mass destruction.

However, when a popular revolution broke out against his father, he warned that the country would burn if it tried to oust the dictator and was accused of playing a significant role in the crackdown against protesters. “We fight here in Libya, we die here in Libya,” he told Reuters in 2011.

His father was eventually toppled with assistance from Nato, and killed in 2011, ending four decades of rule. The country has since been consumed by fighting between different militias and remains divided 15 years later, with two rival governments controlling different parts of the country.

Saif al-Islam Gaddafi tried to flee Libya for neighbouring Niger in 2011 but was caught by a militia and was imprisoned for six years in Zintan, before being freed in 2017 as part of an amnesty deal.

When Gaddafi was captured in the Sahara in 2011 after months on the run, the figure known for his jeans and sweater had a thick black beard and was wearing flowing khaki robes – dressed to blend in with the nomads who were hiding him. The fighters from Zintan, a rebel-bastion town, had intercepted him after a tipoff, and flew him back to their home town on an old Libyan military transport plane whose pilot had defected from the air force.

Gaddafi was quiet on the flight and held his bandaged right hand, which he said had been injured in a Nato airstrike. Abdullah al-Mehdi, the pilot, said he had given Gaddafi his word that he would not be hurt. Saif’s father had been violently tortured and killed after capture. “I spoke to him like he was a small child,” al-Mehdi said at the time.

Four years after he was released, Gaddafi announced himself as a candidate for Libya’s 2021 presidential elections. The announcement provoked outrage from those who had suffered under his father’s dictatorship, and from anti-Gaddafi militias. Rebel groups rejected his candidacy and he was disqualified owing to his 2015 conviction of war crimes, with the election ultimately collapsing in the end.