“He was killed today at 2pm (1200 GMT)… in Zintan in his home by a four-man commando,” Ceccaldi said.
Seif al-Islam, 53, had been seen by some as his father’s successor, despite being targeted by a warrant from the International Criminal Court for alleged crimes against humanity.
In 2021 he announced he would run for president, but those elections were indefinitely postponed.
His adviser, Abdullah Othman Abdurrahim, told Libya’s Al-Ahrar television channel that the four unidentified men had stormed the home before “disabling surveillance cameras, then executed him”.
It remained unclear who was behind the killing.
Ceccaldi said that a close associate of Seif al-Islam had told him days ago “that there were problems with his security”.
“So much so that the head of the (Gaddafi’s) tribe had called Seif and told him ‘I will send you people to ensure your security’. But Seif refused,” he said.
While Seif al-Islam held no official position in the North African country under his father’s rule, he had been described as Libya’s de facto prime minister, cultivating the image of a moderate and reformer prior to the 2011 Arab Spring revolt.
But that reputation soon collapsed when he promised “rivers of blood” in the face of the uprisings.
A divided country
Seif al-Islam was arrested in November 2011 in southern Libya following a warrant issued by the International Criminal Court in The Hague.
A Tripoli court later sentenced him to death in 2015 after a speedy trial, but he was granted amnesty.
His whereabouts had long been unknown. Ceccaldi said he “often moved around”.
Libya expert Emadeddin Badi said Seif al-Islam’s death was “likely to cast him as a martyr for a significant segment of the population, while also shifting electoral dynamics by removing a major obstacle to presidential elections”.
“His candidacy and potential success had been a central point of contention,” Badi wrote on X.
Muammar Gaddafi’s last spokesman, Moussa Ibrahim, also posted on the social network: “They killed him treacherously. He wanted a united, sovereign Libya, safe for all its people.
“I spoke with him two days ago. He spoke of nothing but a peaceful Libya and the safety of its people.”
Libya has struggled to recover from chaos that erupted after a Nato-backed uprising in 2011 overthrew longtime ruler Moamer Gaddafi.
Libya remains divided between a UN-backed government based in Tripoli and an eastern administration backed by Haftar.