The presidential candidate of the Peoples Democratic Party, Atiku Abubakar, was absent at the signing of a peace pact for 2019 presidential candidates in Abuja.
It was not immediately clear why Mr Abubakar, the major challenger to President Muhammadu Buhari, did not turn up at the event.
Mr Abubakar was among those expected at the event.
Spokespersons for Mr Abubakar and the PDP separately told PREMIUM TIMES they were not aware of the event, which began at 3:00 p.m., and promised to make urgent findings and revert.
Mr Buhari arrived at the International Conference Centre to join other candidates in signing the peace agreement, which was drafted by the National Peace Committee.
Abdulsalam Abubakar, a former Nigerian military ruler, is the convener of the committee and the event.
A similar pact was signed by presidential candidates in 2015 and the committee was widely deemed crucial to the peaceful outcome of that contentious election.