ExxonMobil’s deal to secure some oil blocks in Nigeria is under investigation by the Economic and Financial Crimes Commission, according to documents obtained by The Guardian of the United Kingdom.

ExxonMobil, the world’s largest oil company, was said to have secured the lucrative oil rights in 2009 by beating out China’s fourth-largest oil producer for access, despite apparently underbidding its rival by $2.25bn.

The Guardian said a letter addressed to an ExxonMobil’s subsidiary from the Federal Ministry of Petroleum Resources showed the acceptance of a 2009 bid of $1.5bn for a 20-year lease on the Oso, Ekpe, Edop and Ubit oil fields, which produce about 580,000 barrels per day between them – close to a third of Nigeria’s crude oil production of about 1.8 billion bpd, according to the Organisation of Petroleum Exporting Countries.

Sunrise Power & Transmission, which was then a consortium of Nigerian and Chinese interests that included the Chinese National Offshore Oil Corporation, bid $3.75bn for the same rights, according to a letter from Sunrise to the President.

The deal was said to have been reported to the Nigerian authorities by the Chairman of the Civil Society Network Against Corruption, Lanre Suraju, who said he was passed documents related to the deal by a “concerned citizen” after his June 2015 petition to investigate the bid was made public. Suraju was the recipient of a letter dated August 17, 2015 in which the authorities confirm they were investigating the deal.

The Guardian said the documents were given to it by the international watchdog group, Global Witness, which said it had confirmed the investigation with the EFCC, adding that the anti-graft agency said it could not comment on the existence of an investigation, though Suraju said that as of last week, “the EFCC has progressed impressively on the matter”.

It was said that an internal memo signed by executives of the NNPC showed Exxon’s local subsidiary, Mobil Producing Nigeria, originally tried to acquire a 25-year licence to receive billions of dollars of oil for just $75m, saying they had expected the price for the lease renewal to be “millions not billions.”

The three oil-mining leases, renewed in 2009 at the end of a 40-year lease, are for shallow-water offshore lots of oil-rich real estate labelled blocks 67, 68 and 70.