Nigeria signed a $6 billion loan and currency swap deal with China during Nigerian President Muhammadu Buhari's visit to Beijing in early April. The Nigerian anti-corruption agency has discovered over 37,000 ghost workers that have cost the government over $5 million so far. A video released by Boko Haram showed a number of girls identified by relatives as members of the Chibok schoolgirls kidnapped in 2014. Pirates hijacked a chemical cargo ship off the coast of Nigeria and kidnapped six of the crew members. The US released an estimate of the impact of piracy, theft, and fraud in the Nigerian oil industry at $1.5 billion per month. A gun battle between the 113 Battalion of the Nigerian Army and Boko Haram militants broke out on 23 April in Kareto, Borno State. The Nigerian military announced the capture of four high level Boko Haram kingpins. An Amnesty International report revealed that the Nigerian army has been accused of massacring over 350 civilians who were discovered in a mass grave. Dangote Cement announced the construction of a $1 billion cement factory in southern Nigeria.