Wednesday, July 8, 1998 Published at 00:54 GMT 01:54 UK
Abiola death sparks unrest
Chief Abiola's release was believed to be imminent
The jailed Nigerian opposition leader Moshood Abiola has died, one day before his expected release after four years of solitary confinement.
The un-named official said: "The US has no reason to doubt that the sudden death of Chief Abiola was due to natural causes."
The Nigerian authorities have announced that a full post-mortem will be held into how he died.
Pictures released of Chief Abiola meeting the UN Secretary-General, Kofi Annan, last week, showed him in apparent good health, although he had lost weight. He has suffered bouts of illness in jail.
The BBC Lagos correspondent says Nigerians are shocked and suspicious, after the sudden death only a month ago of the military leader, General Sani Abacha.
Chief Moshood Abiola was the presumed winner of Nigeria's 1993 elections, which were annulled by Nigeria's military government.
A year later Chief Abiola was arrested and had been in detention ever since. His release had been widely predicted for Wednesday - when official mourning ends for the Nigerian military ruler who jailed him, Sani Abacha.
It is believed that the government was trying to get Chief Abiola to give up his presidential mandate in exchange for his freedom, although as of a few days ago he had not committed himself to this in writing.
Transition to democracy
Before their meeting with Chief Abiola, the American delegation held talks with Nigeria's new leader, General Abdulsalam Abubakar.
A statement from the Nigerian side said General Abubakar had given assurances about the transition to democracy.
Last week, his government promised to release all political prisoners, including Chief Abiola.