Saif al-Islam, pictured in March, is one of three regime figures, including his father, wanted by the ICC [Reuters]
Libyan rebel forces have arrested a third son of Libyan leader Muammar Gaddafi, after two others were earlier taken into custody by the opposition.
Monday's capture of Saadi Gaddafi, in the capital Tripoli, comes as the whereabouts of other relatives and senior Libyan officials remains unknown.
Earlier, Gaddafi's second-eldest son, Saif al-Islam, was detained by rebels on Sunday night, according to Mustafa Abdul Jalil, the head of the rebel National Transitional Council (NTC). The council provided few other details.
Hours later, a spokesman for the International Criminal Court (ICC) confirmed the arrest. The opposition has not announced whether it will hand over prisoners to the ICC.
The ICC issued an arrest warrant for Saif al-Islam in June on charges of crimes against humanity. Luis Moreno-Ocampo, the court's chief prosecutor, described Saif al-Islam as Libya's "de facto prime minister", and accused him of recruiting foreign mercenaries that reportedly attacked protesters during Libya's six-month uprising.
"We hope he can soon be in The Hague," Moreno-Ocampo said on Monday.
Still, the rebels have not yet said whether they will transfer Saif al-Islam to the ICC. Representatives from the court and the rebel council are scheduled to meet on Monday to discuss a possible transfer.
Eldest son 'unharmed'
Muammar Gaddafi's eldest son, Mohammad, was also detained by rebels on Sunday night.
Mohammad Gaddafi's interview with Al Jazeera
In an interview with Al Jazeera after he surrendered, Mohammad expressed his "sadness" at the fighting in Libya. The interview was interrupted by gunfire.
"What's happening in Libya is very upsetting. The killing between brothers, between Muslims, is something that saddens me," he said, shortly before gunfire rang out in the background.
Abdul Jalil said that Gaddafi was unharmed, though there was no way to verify that claim. Mohammad did, however, conduct another interview on air with Al Jazeera shortly after the gunfire. His whereabouts are currently unknown.
Mohammad was the chairman of Libya's main state-run telecommunications firm, but his role in his father's government was reportedly minimal, far smaller than Saif al-Islam's.
Gaddafi, Senussi fled Tripoli?
Other former senior Gaddafi officials remain at large, including three of his seven sons.
The biggest question is Muammar Gaddafi himself, last heard in a brief audio recording on Sunday night. He called on Libya's tribes to March on the capital.
"How can you allow Tripoli to be burned?" he asked.
It is unclear whether Gaddafi is still in Tripoli, though that seems increasingly unlikely. Gaddafi, like Saif al-Islam, is wanted by the ICC.
Three other Gaddafi sons - Hannibal, Mutasim and Khamis - have not been located. Hannibal had little role in politics, but Khamis headed a feared army unit that took a leading role in suppressing protests. Mutasim was an army officer and a security adviser to his father.
The Al-Arabiya news network reported on Monday that Khamis was traveling to central Tripoli with soldiers loyal to him. That report could not be immediately confirmed.
Abdullah al-Senussi, Gaddafi's longtime intelligence chief and brother-in-law, also seems to have eluded the rebels. He was last seen at Tripoli's Rixos Hotel on Sunday, when he told foreign journalists that "Western intelligence" was "working alongside al-Qaeda to destroy Libya".
There are rumours in Arabic newspapers that Senussi fled Tripoli, either to southern Libya or to the Tunisian town of Djerba, but those reports cannot be substantiated.
Senussi was the third Libyan official charged by the ICC in June. The court accused him of carrying out a campaign of murder, mass arrest and torture.