Libya is set to question Abdullah al-Senussi, Muammar Gaddafi's former spy chief, following his extradition from Mauritania.
Thursday's announcement was met with calls by the United States and international rights groups that Senussi be granted a free and fair trial.
"It will be critical that Libya take all necessary steps to ensure that he's held securely, treated humanely and tried fairly in full compliance with Libya's international obligations," Patrick Ventrell, US state department deputy spokesman, told journalists.
Senussi, former head of military intelligence in Libya and one of the most feared men in Gaddafi's government, is wanted by the International Criminal Court in the Hague, where he is accused of crimes against humanity.
Senussi was extradited from Mauritania on Wednesday, after his March arrest for illegally entering the West African country.
Senussi was discovered in Mauritania as he tried to enter the country from Morocco using a Malian passport under a different name..
Washington swiftly urged Libya to ensure that the former spy chief gets a fair trial, but stopped short of insisting he be handed over to the ICC.
"We think it's important that he's held to account, whether that's in a Libyan setting or otherwise," Ventrell added.
However, rights watchdog Amnesty International said Senussi should have been surrendered to the international court.
"The decision to send him to Libya - with its weak justice system and inadequate fair-trial guarantees - will inevitably delay justice for victims and could lead to violations of Senussi's rights to a fair trial," Marek Marczynski, head of the London-based rights group's International Justice campaign, said in a statement.
In the capital Tripoli, deputy prosecutor general Taha Baara confirmed to the AFP news agency that Senussi was back in the country.
"A short time ago he was delivered to the office of the prosecutor general. He will undergo routine medical examination before questioning begins immediately afterwards," he said.
"We will decide his fate afterwards."
"His extradition took place following the decision of the Mauritanian courts and its ratification by Mauritanian President Mohamed Ould Abdel Aziz," Baara said.
Wednesday's move came after Libya said Gaddafi's son Saif al-Islam would go on trial this month in the town of Zintan, despite an ICC warrant against him also for arrest on charges of crimes against humanity.
Activists have raised concerns that Saif, 40, could face the death penalty if tried in Libya. Saif's lawyers said on July 31 he wants to be put on trial in The Hague for justice to be served.
The ICC says Senussi played a "crucial" role in attempting to crush the popular revolt that eventually ousted the Gaddafi regime late last year.
In the arrest warrant issued in June 2011, the ICC said Senussi was an "indirect perpetrator of crimes against humanity, of murder and persecution based on political grounds" in the eastern Libyan city of Benghazi, cradle of the revolt.
On Wednesday the ICC told the AFP news agency: "We have not received official information on the transfer" of Senussi.
"Libyan authorities have an obligation to surrender Abdullah al-Senussi to the ICC. Abdullah al-Senussi is wanted by the ICC on the basis of an arrest warrant issued by ICC Pre-Trial Chamber for alleged crimes against humanity [murder and persecution] on 27 June 2011," the court said.
Tripoli had pushed hard for the extradition of Gaddafi's brother-in-law who is also wanted by France.
A delegation from Libya, including the defence minister and army chief of staff, was in the capital Nouakchott on Tuesday for a visit that several official sources said was in connection with the extradition.
In 1999, a Paris court sentenced Senussi in absentia to life in prison for involvement in the bombing of a French UTA airliner over Niger in September 1989.
Interpol had issued a so-called "red notice" for Senussi on behalf of Libya. It said he was wanted "for fraud offences including embezzling public funds and misuse of power for personal benefit".
Senussi was one of the last members of Gaddafi's inner circle to be arrested. Others still at large include the former leader's son Saadi, who has taken refuge in Niger.
The ex-spy chief, in his early sixties, had been on the run for months when he was caught trying to sneak into Mauritania.