In the capital, Information Minister Mahmoud Shammam said former Intelligence Minister Abdullah al-Senoussi was captured Sunday by revolutionary fighters from a southern region called Fazan, not far from where Gadhafi's son was seized while trying to flee to neighboring Niger.
Fighters tracking Senoussi for two days caught up with him at his sister's house in Deerat al-Shati, about 40 miles south of the desert city of Sebha, said fighter Abdullah al-Sughayer. Few other details were immediately available about his capture, and it was not clear whether his captors would also resist turning him over to Tripoli.
Although they are wanted by the International Criminal Court in the Hague, Netherlands, Libya is likely to seek to try both men at home.
Speaking earlier in the day, before Senoussi's capture, the information minister said Seif al-Islam, the ousted Libyan leader's onetime heir apparent, must be tried in Libya even though the country's new leaders have yet to establish a court system.
"It is only fair for the Libyan people that he is tried here," Shammam said. "Seif al-Islam committed crimes against the Libyan people."
The ICC indicted the two men in June, along with Moammar Gadhafi, for unleashing a campaign of murder and torture to suppress the uprising against the Gadhafi regime that broke out in mid-February.
Senoussi, Gadhafi's brother-in-law, was also one of six Libyans convicted in absentia and sentenced to life in prison in France for the 1989 bombing of a French passenger jet over Niger that killed all 170 people on board.
ICC spokesman Fadi el-Abdallah said Libya would have to convincingly lay out its arguments in what is called a "challenge of admissibility" if it wants to try the two men at home instead of sending them to the Hague.
"The issue is that there is already a case before" that court, he said. Libya "will need to show that they have a serious, genuine legal system capable of functioning fairly in this case."
Seif al-Islam, who was once the face of change in Libya and who led his father's drive to emerge from pariah status over the last decade, was captured by fighters from the small western mountain town of Zintan who had tracked him to the desert in the south of the country.
He was then flown to Zintan, 85 miles southwest of Tripoli, where he remains in a secret location.
On Sunday, the fighters holding Seif al-Islam posted a video on YouTube of him saying an injury to his hand was a result of a NATO air strike a month ago that struck his convoy in Wadi Zamzam, about 90 miles southeast of Tripoli. He said 26 people were killed in the strike.
Photos of him with three fingers of his right hand in bandages had raised questions about whether he was mistreated by his captors.
In the video, he appeared in good health. He seemed confident, even referring to those holding him prisoner as "brothers and family."