Libya's Presidential Council head Mohammed al-Menfi and Attorney General Siddiq al-Sour on Wednesday vowed investigations into the transfer of Lockerbie suspect Abu Agila Mohammad Mas'ud Kheir Al-Marimi to the United States.
Mas'ud's sudden arrival in the United States after his family said he was seized from their home by gunmen last month has caused widespread anger in Libya at the Tripoli government.
While the United States announced on Sunday that Mas'ud was in its custody and put him in a courtroom on Monday, Libya's Government of National Unity (GNU) has made no comment on the case.
Mas'ud is accused of being a former Libyan intelligence officer responsible for making the bomb that blew up Pan Am flight 103 over Lockerbie, Scotland, in 1988, killing 270 people.
Menfi has demanded "necessary measures according to jurisdiction" in answer to a formal complaint made against the GNU by Mas'ud's family, letters shared by Menfi's spokesman showed.
The head of the Presidential Council, which functions as Libya's head of state, also demanded that GNU Prime Minister Abdulhamid al-Dbeibah tell the truth about any coordination with the United States.
Mas'ud's nephew Abdulmeman Marimi told Reuters on Monday that his uncle had informed them he was being held by the Joint Operations Force in Misrata, a security unit seen as close to Dbeibah.
The family has complained that Mas'ud's extradition was made "without judicial procedures".
Critics of Dbeibah accuse him of trading Mas'ud to the United States to curry its support in his political standoff with an eastern-based parliament that backs an alternative administration.
Armed factions are split between the two sides but despite sporadic outbreaks of fighting there has been no return to the open warfare that last ended in summer 2020.
However, both sides have tried to court public opinion inside Libya and the backing of foreign powers who could help them gain or hang onto office.
While the three-man Presidency Council that Menfi heads was appointed at the same time as the GNU through an identical U.N.-backed process early last year, it has tried to stay out of the political standoff.
Under the terms of an internationally recognised political agreement in 2015, Sour can only be replaced as attorney general if both the parliament and a rival institution, the High State Council, agree.
(Reporting by Reuters Libya newsroom; Writing by Angus McDowall, Editing by Alexandra Hudson)