US President Donald Trump, in his strongest remarks so far, said Crown Prince Mohammed bin Salman bears ultimate responsibility as de facto ruler of Saudi Arabia for the operation that led to the death of Saudi journalist Jamal Khashoggi, indicating that the prince is "running things over there" in Riyadh.Speaking to the Wall Street Journal on Tuesday, Trump said he wanted to believe the prince's claim that lower level officials were to blame for the Oct. 2 killing at the Saudi consulate in Istanbul, but suggested responsibility lay higher up.Asked about Prince Mohammed’s possible involvement, Trump said: "Well, the prince is running things over there more so at this stage. He's running things and so if anybody were going to be, it would be him."Trump said he was convinced King Salman had no advance knowledge of the incident, according to the Journal.Earlier on Tuesday, the president told reporters at the White House that Saudi authorities had staged the "worst cover-up ever" over the incident, and that they had handled the matter badly.The brutal death of Khashoggi, a US resident and Washington Post columnist critical of the crown prince, has sparked global outrage and threatened relations between Riyadh and Washington as well as other Western nations.In his interview with the Journal, Trump said he had asked the prince multiple times about the matter."My first question to him was, 'Did you know anything about it in terms of the initial planning?'" Trump told the Journal, adding that the prince told him that he did not.The president said he asked the prince "'Where did it start?' And he said it started at lower levels."Asked if he believed the denials, Trump told the newspaper: "I want to believe them. I really want to believe them," he said.The United States also on Tuesday vowed to revoke the visas of some of those believed to be behind the attack.Turkey, which is investigating the killing, has dismissed Saudi efforts to blame rogue operatives and has promised to hold those behind Khashoggi's death responsible.