Senussi is wanted by the Hague-based International Criminal Court for crimes against humanity but was extradited to his home country on Wednesday.
The fugitive's extradition from Mauritania, where he was arrested in March as he tried to enter the country from Morocco using a Malian passport under a different name, was first announced by Mauritanian state television.
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.
"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," US State Department deputy spokesman Patrick Ventrell told journalists.
"We think it's important that he's held to account, whether that's in a Libyan setting or otherwise," he added.
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," Amnesty's Marek Marczynski said in a statement.
Back in the country
In Tripoli, deputy prosecutor general Taha Baara confirmed 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."
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 Seif (40) could face the death penalty if tried in Libya. Seif'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, the former head of military intelligence in Libya, 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 said: "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 and feared former right-hand man, who is also wanted by France.
A delegation from Libya, including the defence minister and army chief of staff, was in 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.
The plane was carrying 170 people from Brazzaville to Paris via N'Djamena.
That attack – along with the bombing of a Pan Am jumbo jet over Lockerbie, Scotland in December 1988 in which 270 people were killed – led to a UN-mandated air blockade of Libya in 1992.
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 Gaddafi's son Saadi, who has taken refuge in Niger.
The ex-spy chief, a heavy-set man in his early 60s with a thick jaw line and bushy black eyebrows, had been on the run for months when he was caught trying to sneak into Mauritania. – Sapa-AFP