Liberia's ex-President Charles Taylor, who is wanted on war crimes charges, has disappeared from the villa where he lived in exile, Nigeria says.
Charles Taylor stepped down in 2003 to end Liberia's civil war
All of those supposed to have been guarding him have been detained.
At the weekend, Nigeria said it would let him be arrested, but both Liberia and the US said Nigeria should send him to a UN-backed war crimes court.
Mr Taylor stepped down as president in 2003 under a deal to end the Liberian civil war, which he started in 1989.
He went into exile in Calabar, in south-eastern Nigeria.
The BBC's Mark Doyle in Freetown says there are powerful political forces at play over Mr Taylor's fate.
Mr Taylor's whereabouts are not clear but his spiritual advisor Kilari Anand Paul told the BBC News website that Mr Taylor was in the Liberian bush, from where he first launched his rebellion.
He also said the former Liberian leader would be happy to face justice in The Hague but not in Sierra Leone.
The AP news agency reports that members of Mr Taylor's entourage have been leaving Calabar for several days.
Nigeria's President Olusegun Obasanjo is "very shocked" by Mr Taylor's disappearance on Monday, Information Minister Frank Nweke told the BBC's Focus on Africa programme.
Nigeria has set up a panel to investigate the matter, and to establish whether he escaped or was abducted, Mr Nweke said.
Lobby group Human Rights Watch says Mr Taylor's disappearance is a "disgrace".
"It brings into question President Obasanjo's commitment to peace and security for the people of West Africa, the reason he allegedly took Taylor in the first place."
After Nigeria announced that it would let Mr Taylor face trial, Desmond de Silva, chief prosecutor of the war crimes court in Sierra Leone, called for Mr Taylor's immediate arrest, warning that he could use his vast wealth and contacts to organise his escape.
He described Mr Taylor as one of the three most important wanted war crimes suspects in the world.
Mr Taylor was indicted on 17 charges of war crimes and crimes against humanity, concerning his alleged backing for Sierra Leone's rebels, shortly before stepping down in 2003.
1989: Launches rebellion
1991: RUF rebellion starts in Sierra Leone
1995: Peace deal signed
1997: Elected president
1999: Lurd starts rebellion to oust Taylor
June 2003: Arrest warrant issued
August 2003: Steps down, goes into exile in Nigeria
The news comes as a huge embarrassment for Mr Obasanjo, who is travelling to the US to meet President George W Bush in Washington on Wednesday.
On Monday, the US said it was "incumbent upon the Nigerian government now to see that he [Mr Taylor] is conveyed to the international court."
Liberian President Ellen Johnson-Sirleaf said she wanted her predecessor to be sent directly to Sierra Leone, as he had not been indicted by a Liberian court.
A number of Mr Taylor's supporters have been detained in Liberia amid fears they may stage an armed uprising.
Tens of thousands of people died in the interlinked conflicts in Sierra Leone and Liberia.
Mr Taylor is accused of selling diamonds and buying weapons for Sierra Leone's Revolutionary United Front rebels, who were notorious for hacking off the hands and legs of civilians during a 10-year war.
He also started the Liberian civil war in 1989, before being elected president in 1997.