A BBC investigation has found evidence suggesting some results from Nigeria's presidential election may have been manipulated.

The winner Bola Tinubu is due to be inaugurated on 29 May but the opposition is challenging this.

The BBC has uncovered significant anomalies in Rivers state, a key battleground, although not sufficient to change the overall national outcome of the election, which took place in February.

There are also questions over the identity of an election official who read out some of the unexplained results.

Two staff members of the U.S. Consulate in Nigeria were among the four people killed by armed assailants when an American convoy was attacked on Tuesday, the State Department said.

The staff members, who were not U.S. citizens and who haven't yet been publicly identified, were killed by people who were "linked with criminal groups" in the country's southeast, U.S. officials said.

Two members of a local Police Mobile Force were also dead following the assault, Nigerian police officials sad. The victims' bodies were later set on fire, officials said.

Ex-Georgia Democratic gubernatorial candidate Stacey Abrams, who has denied election results in America, was roasted by conservatives this week when it was revealed she was in Nigeria to serve as an "international observer" to the nation’s presidential election. 

Bola Ahmed Tinubu has been declared the winner of Nigeria's fiercely contested presidential election but many, including opposition parties, have claimed the poll was rigged and a sham.

They have called for the election to be cancelled, alleging breaches of the Electoral Act and possible collusion by the Independent National Electoral Commission (Inec) to doctor the result in Mr Tinubu's favour.

The allegations have been denied by Inec and Mr Tinubu's party, the ruling All Progressives Congress (APC).

Ruling party candidate Bola Tinubu was declared the winner of Nigeria's presidential election early Wednesday, amid calls from the opposition for a rerun in Africa's largest democracy.

Nigeria’s Catholic bishops have warned that the nation could plunge into “an avoidable crisis” if trust in the electoral process is not restored after this week’s dubious election.

On Wednesday, Nigeria’s Independent National Electoral Commission (INEC) declared Bola Ahmed Tinubu the winner of a controversial presidential ballot vote, amid accusations of rigged elections and calls for a return to the polls.

Nigeria's ruling party candidate Bola Tinubu has an unassailable lead in a disputed presidential election held over the weekend, a Reuters tally of provisional results from all 36 states and the federal capital Abuja showed on Tuesday.

In the past few weeks, as Nigeria’s Feb. 25 elections approached, the country seemed to be in the middle of an economic and infrastructural meltdown. The Central Bank of Nigeria had recently recalled the three largest-denomination naira notes, requiring that more than 200 million people deposit all their old notes within a span of three months, in a country where people mainly use cash and where millions do not have bank accounts at all.