President Trump said Tuesday that Saudi Arabia has conducted one of the "worst in the history of cover-ups" after the death of Saudi dissident Jamal Khashoggi.

"They had a very bad original concept, it was carried out poorly, and the cover up was the worst in the history of cover ups," Trump told reporters in the Oval Office. "They had the worst cover-up ever."

"Whoever thought of that idea, I think, is in big trouble. And they should be in big trouble," Trump added.

Khashoggi, a Washington Post columnist, was last seen the afternoon of Oct. 2 entering the Saudi mission. Turkish investigators believe he was murdered inside the embassy, where he was to fill out paperwork for his wedding, and previously told U.S. officials they have audio and video evidence to prove it. Turkish officials claim Khashoggi was dismembered at the consulate.

Saudi Arabia's government has denied Crown Prince Mohammed bin Salman had any involvement in Khashoggi's death, clashing with claims by the Turkish government that the crown prince ordered a hit on the journalist.

Saudi Arabia has changed its story over time, first saying that Khashoggi had left the consulate alive, but later admitted he was dead, claiming he was killed following an interrogation that “developed in a negative way, leading to a fistfight.”

[Opinion: On Khashoggi, the Saudis have a new story with a glaring hole]

The Saudi government, which has said it is conducting its own investigation, has fired five Saudi officials and arrested 18 others.

Until Trump's comments Tuesday, the president's most direct criticism of the Saudis on Sunday. Trump told reporters reporters that "obviously there has been deception, and there's been lies." Trump previously floated the idea that Khashoggi was murdered by "rogue actors," which Saudi Arabia's foreign ministry began parroting Sunday.

Bin Salman claims his government has "nothing to hide" in relation to Khashoggi's disappearance.

Senior White House adviser Jared Kushner, who has met with Crown Prince Mohammed on a number of occasions and is reportedly close to him, said Monday that he told the crown prince "to be fully transparent," because "the world is watching."

Secretary of State Mike Pompeo traveled to Riyadh early last week to meet with the Saudi royals. The secretary said that the king and crown prince "assured me that they will conduct a complete, thorough investigation of all of the facts surrounding Mr. Khashoggi and that they'll do it in a timely fashion."

Meanwhile, CIA Director Gina Haspel traveled to Turkey on Monday to address its investigation into Khashoggi's death.

In a speech Tuesday, Turkish President Recep Tayyip Erdogan said the killing was "planned" and demanded that Saudi Arabia release all the names and ranks of those involved.