"They had a very bad original concept. It was carried out poorly and the cover-up was one of the worst in the history of cover-ups," Trump declared Tuesday in the Oval Office.
"Bad deal, should have never been thought of. Somebody really messed up. And they had the worst cover-up ever," he added.
"Because whoever thought of that idea, I think is in big trouble. And they should be in big trouble. Okay?"
After more than two weeks of near silence, Saudi Arabia admitted Saturday that Khashoggi, 59, was killed in its Istanbul consulate on October 2, in what it said was an operation gone wrong.
But Turkey's President Recep Tayyip Erdogan charged Tuesday that Khashoggi was targeted in a meticulously planned killing -- and demanded that 18 suspects detained by Saudi Arabia be extradited to Istanbul to face trial.
Asked about Erdogan's call for those behind Khashoggi's "savage murder" to face punishment, Trump said the Turkish president had been "pretty rough on Saudi Arabia."
But when questioned about whether he believes the Saudi account of events leading up to Khashoggi's death, Trump replied, "I want to see the facts first" -- while reiterating Riyadh's importance as a US ally in the Middle East.
"Look, Saudi Arabia has been a really great ally. They've been one of the biggest investors, maybe the biggest investor, in our country," he said.
A former royal family insider turned critic of the Saudi crown prince, Khashoggi disappeared after he entered the consulate to collect a document for his upcoming marriage.
The murder of the Washington Post contributor has severely dented the international reputation of Crown Prince Mohammed bin Salman. The killing also overshadowed a major investment forum that opened in Riyadh Tuesday.