SAO PAULO (AFP) – Brazil on Wednesday sought to uncover the cause of a massive and mysterious blackout overnight, amid concerns of energy supply stability for the 2016 Olympic host nation.

The outage, which hit at 10:15 pm Tuesday (0015 GMT Wednesday) and lasted four to six hours, plunged nearly half the country into darkness.

An estimated 50 million people -- more than a quarter of Brazil's 190-million-strong population -- were affected, mainly in the major cities in the southern part ...