"No way now to tolerate Saudi whitewash or cover up. All culpable, including the Crown Prince, should be held accountable," Blumenthal wrote.
The senator stressed that President Donald Trump and the US Congress should unite on the issue and speak to Saudis from a position of strength, demanding a thorough and immediate investigation.
An independent probe conducted by US experts is needed because the kingdom has tried to change the narrative and has contradicted its previous statements, Blumenthal noted.
The senator said the murder was planned by the crown prince to suppress free speech in the country.
CIA Director Gina Haspel headed to Turkey late Monday to assist in the probe amid scrutiny over Prince Mohammed's involvement in the case, US media reported.
Following more than two weeks of denials, Saudi Arabia admitted on Friday via state television that Khashoggi was killed in a brawl inside the Saudi Consulate in Istanbul on October 2. According to Riyadh, 18 people have been arrested over their involvement in the killing. Turkey is conducting a separate investigation into the journalist's death.