New York - The United Nations raised questions on Tuesday about the Democratic Republic of Congo's probe of the murder of two UN experts investigating mass graves, saying it appeared to have been done in haste.Congolese authorities on Saturday said they had completed a 10-week investigation and that two men will face trial for the murders in March of American Michael Sharp and Swedish-Chilean Zaida Catalan in central Kasai province."That seems to have been done with quite a bit of rapidity," said UN spokesperson Stephane Dujarric, adding that the Kinshasa government had yet to share the findings of their investigation with the United Nations.The UN Security Council met behind closed doors, at the request of the UN secretariat, to discuss the death of the two members of the UN panel of experts investigating mass graves in Kasai.Swedish diplomat Carl Skau said the United Nations must step in to ensure that the murders are properly investigated."We will be asking the UN to put options on the table," Skau told reporters. "It's been obvious in the last couple of days that more needs to be done to ensure that this is done in a proper manner."A board of inquiry set up by the United Nations will present a report at the end of July, Dujarric said.Swedish and US investigators are also looking into the case.In Kinshasa, the Congolese public prosecutor said earlier that he was investigating allegations that former development minister Clement Kanku may have been linked to the murders.In a telephone conversation with an alleged militia fighter, Kanku - now an opposition MP - seems to be discussing setting fire to a town as well as killing officials, according to a report by the New York Times.Kasai has seen a major spike in violence since September when government forces killed Kamwina Nsapu, a tribal chief and militia leader, who had rebelled against President Joseph Kabila.