Trump and Secretary of State Michael Pompeo initially placed inordinate weight on Saudi denials that the kingdom was responsible for Khashoggi's disappearance, and the president sought to downplay the affair.
It, however, said the investigations were still in the process and that 18 Saudi nationals were being held for questioning.
Earlier Friday, Turkish prosecutors questioned staff at the Saudi Consulate, state media said, suggesting attempts to strengthen a possible criminal case with insider details from the last place journalist Jamal Khashoggi was seen alive.
In a stunning confession, the Saudi government has for the first time confirmed that journalist Jamal Khashoggi was indeed killed after a fight inside the Saudi consulate in Istanbul. King Salman, his father, appointed him to lead a committee that will restructure the kingdom's intelligence services after Khashoggi's slaying.
"If the President will not stand up to Saudi Arabia, then we in Congress must stand strong for our nation, our values and journalists throughout the world", Costa said.
The episode had threatened to create a diplomatic crisis with the U.S., which sees its alliance with Riyadh as the cornerstone of its Middle East policy, and Turkey, which has tenuous ties with Saudi Arabia.
"Khashoggi's Turkish fiancee, Hatice Cengiz, tweeted in Arabic: "The heart grieves, the eye tears, and with your separation we are saddened, my dear Jamal", she said, also asking "#where is martyr Khashoggi's body? Republican Sen. Lindsey Graham said on Twitter, "To say that I am skeptical of the new Saudi narrative about Mr. Khashoggi is an understatement".
In the announcement on Saudi State TV, it said a royal order has been issued to release Maj. Khashoggi disappeared after entering the kingdom's consulate in Istanbul Oct. 2. The brawl led to Khashoggi's death and their attempt to hide and cover what had happened.
There has been no indication that Khashoggi had any immediate plans to return to the kingdom. But some USA lawmakers expressed doubt about the Saudi explanation.
Supposedly, a team of 15 Saudis who had arrived in Turkey earlier in the day on two private aircraft, dismembered Khashoggi's body and disposed of it in some as-yet unknown manner.
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Saud Qahtani, a powerful adviser to Prince Mohammed, also was sacked.
King Salman is 82 years old.
Assiri had no immediate comment. Khashoggi had gone to the consulate to get papers he needed to be married.
Turkish police searched a forest on Istanbul's outskirts and a city near the Sea of Marmara for Khashoggi's remains, two senior Turkish officials told Reuters.
Where is the evidence that has been repeatedly referred to by the Turkish authorities?
President Donald Trump has said that the consequences for the Saudis "will have to be very severe" if they are found to have killed him, but has insisted insisted that more facts must be known before making assumptions.
United States officials have told CNN that the operation could not have been carried out without the knowledge of Prince bin Salman, Saudi Arabia's de facto ruler. Trump has yet to nominate an envoy to the kingdom.
He disclosed he would work with Congress to orchestrate an appropriate USA response.
"Saudi Arabia has been a great ally, they've been a tremendous investor in the United States", Trump said, adding, "That's why this is so sad".