Mutassim Gaddafi, one of ousted leader Muammar Gaddafi's sons, has been arrested in Sirte, according to officials in Libya's National Transitional Council.
He is currently being held in Benghazi, the NTC's eastern power base, after being arrested on Wednesday, NTC officials told the Reuters news agency.
"He was arrested today in Sirte," Colonel Abdullah Naker of the Tripoli Revolutionary Council told Reuters.
Fighters in Sirte were reportedly celebrating the news of his arrest on Wednesday night, while citizens in the capital Tripoli took the streets to sound their car horns and fire guns into the sky.
James Bays, reporting from Tripoli, said he had spoken with several high-level NTC officials who had heard the news but could not confirm it. A spokesman for the military said he had not spoken with anyone who had seen Mutassim in custody.
The NTC claimed in August to have captured Gaddafi's highest-profile son, Saif al-Islam, during the final battle for the capital, but that claim turned out to be false, and Saif appeared in public hours later.
If Mutassim was captured, the NTC will be eager to question him regarding the whereabouts of his father and brothers, who are thought to have fled Tripoli as it fell into opposition hands in late August.
Mutassim served as his father's national security adviser while the regime was still in power, but was not seen as being as prominent as Saif al-Islam, Saadi or Khamis, three of his other brothers.
Saif was considered Gaddafi's likely successor, Saadi was known for his failed Italian football career, and Khamis controlled the country's most powerful military unit, which was named after him.
All four, particularly Mutassim, Saif and Khamis, are loathed by many Libyans for the role they played in Gaddafi's authoritarian state and in the crackdown against the revolution.
Saif, Saadi and Khamis's whereabouts are unknown.