The US government says federal agents have raided the Maryland home of Nigerian Vice-President Atiku Abubakar.
Abubakar used to be a close ally of President Obasanjo
The search took place on 3 August, but officials refused to confirm reports that it was linked to raids on homes belonging to a Louisiana congressman.
There has been no comment on the case from Mr Abubakar.
The announcement coincides with remarks by Nigeria's President Olusegun Obasanjo accusing his deputy, formerly his close ally, of doubtful loyalty.
US congressman William Jefferson's properties were searched on the same day.
A Democrat who has served eight terms in the House of Representatives, Mr Jefferson has come under scrutiny as the FBI looks into an international telecommunications deal.
Investigators are interested in Mr Jefferson's links with Mr Abubakar and the vice-president of Ghana, Aliu Mahama, the New Orleans Times-Picayune reports.
According to the newspaper, Mr Jefferson visited Ghana in mid-July.
Although he is technically the second most powerful man in Nigeria, Mr Abubakar has kept a low profile in recent months, says the BBC's Grant Ferrett.
Political manoeuvres within the governing party have undermined his ambitions to stand for the presidency in elections scheduled to take place in two years' time.
On national television and radio, Mr Obasanjo said he had questioned Mr Abubakar's loyalty to his face.
"I brought out a Bible and a Koran and I said... we should not have a house divided. He refused to swear because he obviously knew that the proven cases [of disloyalty] are proven."
Mr Obasanjo denied swearing an oath to his deputy pledging that he would not stand for re-election in two years' time.
He said he had publicly sworn to defend the Nigerian constitution, which prevents him standing for a third term in office, and so did not need to make such promises in private.
These comments have prompted some observers to question whether the FBI investigations are linked to such disagreements.
Mr Abubakar reportedly only uses his home in Potomac, Maryland for a few months each year.
His wife, Jennifer, is a doctoral student in international relations at the American University in Washington.